Welcome to Captive Reefing! I agree with Dakar on the colt, but where did you get the live rock? Was it rock you ordered and had shipped in, or bought personally from an LFS? If the anemone hitchhiked in right where you see him on the rock, neither you or the salesperson noticed him there? The first thing I'd say to you is, FEED HIM! They love Brine Shrimp. You have to buy it from you fish store if they carry frozen foods, or go to another LFS store. Brine shrimp is very digestable for an Anemone. Never feed him a whole fish as they cannot digest them in captivity. Oh, and don't forget to buy a baster. That's a great way to feed them without messing up the tank. Thaw the brine shrimp in some of your tank water & when it's room temperature (don't heat it!), fill the baster with your brine shrimp & water. You shouldn't need much to feed him, and it may take some time for him to figure out the whole feeding game. Cyclopeeze, also bought frozen can be fed as well. Just make sure you shut you skimmer off for a while. Some people do, and some reefers have anemones that prefer them and like to eat your beautiful salt water fish. This anemone looks to small to handle or likely to kill any of your fish. Is your tank completely cycled? Did you or the salesperson see him on the rock since he's obviously got his foot down, and is well anchored to the rock. How long have had the live rock? As far as the anemone goes, I've never seen one quite like that with the exception of a sebae anemone that has been bleached. If it is a sebae, his actual color should be a muddy pink. Do his tips have any coloring to them? It sure is "perfect" and looks very healthy. Are you sure it's alive? I don't mean anything negative by this, but it almost looks like one of those fake anemones that you can purchase for your fish that needs a host.
I have to say, you got awfully lucky if it hitchhiked, unless it's some strange one that's going to nip your fingers off when you feed it!
not to mention if the salesperson saw them, the price of the rock would possibly have gone up dramatically. Around our neck of the woods in WI, you would have had to pay $30+ for both species! Well, good luck. If you have yourself a live one their, there's many of us here who will give you great advice. ....Angel