Looking at some more pictures of Aiptasia I believe that is what you had there. I was a bit confused with the large oral disc, but see that some have that feature and with the curly tentacles I am fairly certain of it being an Aiptasia sp.
"Many aquarists consider Aiptasia to be the dandelion of the ocean. Just like our weeds in our yards, they grow quickly, and will come back if you don't remove the entire organism. If even a portion of the pedal disc (foot that attaches to the substrate) or tentacles remain, it can regenerate a new polyp. Sloppy removal can easily turn one polyp into hundreds in a short time. Like the mythic Greek hydra, when you remove one head, two grow in its place! If the anemone is repeatedly irritated (say, by consistently removing the head-or oral disc), it will asexually reproduce at an alarming rate. Some hobbyists attempt to remove their Aiptasia manually, but it can result in a population explosion. What is even worse, these anemones can "walk" or "swim" away. They will detach their pedal disc and launch themselves into the water, pulsing their bodies like a jellyfish, until they have found a better, more protected area of your aquarium. They can also detach and reattach their pedal disc in a manner that helps them walk along the substrate, just like a snail or corals from the family Xeniidae."