need help with ID please

[tmbar19]

Was given this rock with brown anemonies on it. Apparently they grow very large and reproduce like mad. Looks suspiciously like a kind of aptasia to me. I am afraid to put it in my tank. What do you think? They close up and travel like a regular anemone. (????)

Also, have been trying to catch a 1/2" mantis. I saw it in my tank. Have lost a couple of crabs. Now I'm wondering if the clicking and demolition is from this big green crab? Would a mantis under an inch be able to crush my blue-leg hermits? Should I be getting this big green guy (shell almost 3") out of my tank?
Thanks, Marianne