Confers the ability to change skin pattern to match cover. When a soldier moves to high cover, enemies without special abilities will not target soldier. Does not work if any enemies have line of sight to the soldier's starting position. Leaving cover or firing will break this concealment.
Having Mimetic Skin allows a soldier to more safely traverse large distances on the battlefield by running. This makes it significantly easier to complete time-sensitive objectives like recovering Meld, disabling bombs, and saving civilians in Terror Site missions.
Mimetic skin is one of the most game-changing of gene mods. Creating an entire squad with this mod can give your squad a dramatic edge in combat and therefore should be considered as a priority for Meld usage.
If your entire squad has Mimetic Skin, it is often a good tactic to use a two-phase approach to ambushing enemy alien packs. In the first phase, move all of your soldiers into position so they have good cover and Line of Sight to the enemy; end your turn once all your soldiers are in position. At the start of the next turn, all your soldiers should have their full turn to fire at the enemy alien pack.
If your entire squad is stealthed around an enemy pack, the first shot is the most important for two reasons. First, enemy packs are generally clustered and not in cover; so using explosive/AoE weapons or getting a "free" kill with an In The Zone Sniper both make for great uses of a "first shot". Second, consider that alien packs, once spooked, will take cover from the direction of the soldier who fired the first shot. This means that it is often useful to spook enemy packs with a soldier who is facing the enemy pack from a different angle than the rest of your soldiers' this sets-up the rest of your squad up to get flanking shots.
As long as the soldier is moving into an area with full cover and was not seen by any enemy, Mimetic Skin will automatically activate.
The direction of the full cover is not taken into consideration. Whether or not the soldier is flanked (or will be flanked) has no effect on the activation of Mimetic Skin.
Mimetic Skin will deactivate if a soldier moves after spotting an enemy even though the soldier is still undetected if the soldier and the enemy has the same range of sight, which is mostly the case, because this way the enemy can see the tile where the soldier is at. (One tile behind full cover is seen in the game's mechanics.)
Mimetic Skin will activate on either of a soldier's two available moves; it is possible to use a soldier's first move to break line of sight, then use the second move to reach full cover and activate Mimetic Skin. To break line of sight, you only need to go behind a full cover area which is surrounded on either sides by its equivalents (either cover or circumstantially related solid obstacle) that is oriented in a way to block vision as normal real world relations happen.
Snipers with Low Profile treat partial cover as full cover, thus allowing Mimetic Skin to activate when moving into any cover. This supposedly works with Alloy S.H.I.V. cover also.
Reloading and Hunker Down do not break Mimetic Skin. Overwatch only breaks Mimetic Skin if the soldier actually opens fire.
Ghost Armor's Ghost Mode cannot be activated if the soldier using it has Mimetic Skin. Likewise, Ghost Grenades do not work on soldiers with this gene mod.
Firing a weapon while concealed by Mimetic Skin confers the same +30% critical hit chance that Ghost Mode does.
Archangel Armor's Flight does not count as a valid cover. A Sniper flying with Low Profile will not trigger Mimetic Skin.
However, if a unit is flying and lands in heavy cover while the unit is not sighted, Mimetic Skin will activate on the way to the ground.
If the cover of a cloaked soldier is destroyed (or reduced to half-cover), Mimetic Skin will immediately deactivate. This possibly does not happen with a Sniper.
If an Assault activates Run and Gun at any time while still visible, they will not cloak even if all the standard rules mean that they otherwise would.
It is often more beneficial for a cloaked soldier to do nothing when in sight of enemies, so that the End Turn command (by default the Backspace/End key) can be more useful than Overwatch (since the soldier might open fire) or Hunker Down (since the soldier loses sight).
Soldiers with the Close Combat Specialist perk therefore MIGHT make poor choices for this gene mod, as they will automatically attack any foes who close the requisite distance regardless of their Overwatch status – in turn immediately distrupting their cloak. However this can be hugely planned into any strategy.
A way to work around this is to command such soldiers to end every turn with Hunker Down – or to send them into situations of expected enemy proximity with an empty weapon.
Untriggered enemies may "teleport" to other locations even if your soldiers have sight to them while invisible. Unlike the teleport bug in XCOM: Enemy Unknown, the enemy will only teleport to locations your squad cannot see.
There may be cases where enemies will drop on soldiers that have Mimetic Skin activated. Both the enemy and the soldier will occupy the same tile. Though this may be a bug, Mimetic Skin will not deactivate unless the standard de-cloaking actions are taken.