Rock crab for sure.Not bad when small.Will be a problem when its bigger. We find them all the time since we collect our own live rock. We just smash the big ones and feed to fish, they will go crazy for it.
This type of small crab exists under basically every rock. I'm assuming you grabbed it locally since you live in tampa and these guys are abundant here? This just makes a long list of unappealing little drab critters that will kill something given the chance. The advice remains the same- smash it, feed it to the fish.