They are ambush predators. Using a specially programmed AI, they take the form of the block they're standing on top of, retract their head and legs, and align themselves to Minecraft's block grid. This makes them nearly undetectable in most cases until it spots you.
Once a Chameleon Tick has spotted a player, it will extend its legs and head and make chase, inflicting one heart of damage per hit.
Chameleon Ticks have a weakness in the Elysian Fields; if one is standing on top of a Grass Block, regardless of whether you are using OptiFine and have connected side textures for Grass or not, it will appear as a stock-standard Grass Block of a different shade of green. Using OptiFine makes this even more blatant, as Chameleon Ticks do not adopt the connected side texture for Grass, making them stand right out.