Mysterious Green Worm (?) with red dot ID?

[wrennie]

I saw this green worm looking thing in my tank. It was about 0.5 cm long and about 0.3 cm wide-very rectangular, it didnt appear to have legs, and had a bright red dot on one end of its body right in the middle of it's width (if that makes sense!). It was moving rather slowly, almost like a inch worm, then a crab came up and it curled up and then moved rather quickly out from under the crab.

anyhow, does anyone know what im talking about? Is this thing dangerous? It disappeared anyhow, so I dont think there is much i can do....