Ahhhh, my old friend aptasia
Don't be a gimp like I was and say:
"Oh wow, look what's growing!! Awesome! Everything must be great with the water if fragile anenome things just start growing"
Well, I let them grow and after the first hundred or so I started questioning.
Little too late.
I literally had thousands and thousands and thousands of aptasia completely over running my 300gal.
I wound up transfering livestock to a 55gal and completely gutting/overhauling the tank.
Kill them by any means necessary and keep an eye out for any others CONSTANTLY.