Unknown Critter

[Hangfire]

Hello All

I've just jumped into the marine aquarium hobby after a few years of research. The tank is up and running, and has finally cycled - just put three snails and five hermit crabs in the tank. I've noticed this little critter attached to one of my rocks and was wondering if anyone knows what it is. I have a few of them scattered about and I'm hoping it's nothing bad. Thanks for your help.