What is this?
Any help identifying this little sucker would be appreciated.
Some background... I'm a total newbie to a marine aquarium -- as in this is the first tank and it's been running less than a month. This critter showed up about a week after I put the live sand / live rock in the tank. He planted himself on the glass, front and center of the tank. He's progressively grown from a barely visible spec to ~ 3mm today.
What is it... and more importantly, do I need to do anything about it?