I was always under the impression that aptasia can split like their bigger brother (anemone's). If so you may have just caught it in that stage. And as far as something like that goes, it is probably safer to remove, until you have a posotive ID. From what everyone on here has taught me. If you didn't put it in there then it shouldn't be there. It to me has been a safety measure that I did not listen to in the beginning. Now I am battling the retched aptasia.