Mantis shrimp will also resemble a lobster at the tale, if that helps identify it. You will hear clicking in the tank if it's a smasher. They are pretty interesting, so if you don't have expensive mollusks or bottom dwelling fish to worry about, I would give it a chance to live and try to feed it. They are not always as bad as their reputation claims. The smashers will however bore holes into rock, the only good side here is that they could be partly sustaining on worms within the rocks. Small snails like nassarius get whacked about one per week by the small Caribbean Stomatopods, IME, they will stash their kill for later dining so you don't have to worry about a mass wipeout of inverts. If you see empty nassarius shells decorating the rim of a hole, then you probably have a smasher Stomatopod in your tank.
The flower things sound like colonial feather dusters to me, they're very small, red and always in a group with entangled calcareous tubes. I have never seen any yellow on them though.