Aptasia does come in several forms. Aptasia in general is a cnidarian i think. Its a type of Anemone just like mushrooms are a type of anemone. If it's multiplying then worry. It will eventually spread to other rocks and other coral areas. Just my prediction. All anemones sting corals and you'll probably run into problems down the line. A conventional Anemone is one large piece and while it may be big, won't multiply. Its up to you.