When a non poisonous animal resembles a poisonous animal it's called what?

It's called mimicry. Some non-venomous species adopt similar colour patterns to venomous species. A typical example would be certain members of the (harmless) Milk snake family. They have bands of red, yellow and black colouration arranged in a similar pattern to that of a (venomous) Coral snake. The only difference, is the order the bands are arranged.