so for the past two or three days i have seen a mysterious creature in my tank, every time i see it it is always in the same place but when i get up to see what it is, it darts into a hole in the rock at the slightest movement. today i saw it again and was able to see it was a crab, a black crab. i havent introduced any crabs to my tank in a long time and i have only seen it for the past couple days so i doubt it came in on my live rock a year ago. from what i saw everything about it was characteristic of an emerald crab and i know emerald crabs can come in alot of colors so my question is, is black a color variant of emerald crabs?