The green brittles are known for their predatory nature. Get any other "color" (species, sorry, can't name the species for you at the moment) other than green in the brittle mode and you'll be golden. Other than that, go for SERPENT STARS! I've got a piccie of a red serpent as my wallpaper, and I've decided that it would also make a great tattoo...nothing to do with tanks, but everything to do with beauty, eh?
Yeah, so, a very rough, general rule of thumb is if it's knobbly/bumpy, stay away from it, if it's smooth (as echinoderms go, doncha know) it should be cool.
Other than THAT, be sure to watch water parameters, as sea stars can be on the delicate side.