Grins: you are right. I didn't know what it was so I took the photo to my local reef store and he quickly said....you have zoa eating nudibranch. I need to do some research because he said they were really bad because they lay eggs quickly but this preys only on zoa....and luckily I only have three small polyps.
Wonder if these are harmful to humans? or just the zoas. Reading about nudibranches...some are poisonous.