If you can, see if you can manually remove that one since it looks fairly large. You more than likely have more and would look into getting some peppermint shrimp so those suckers can clean them up!
Sure is, a peppermint shrimp did the job for me as well. I also dose the red sea aptasia a couple of times before, but they pop out again. So I recommend a peppermint shrimp.
Yes. Looking like a single. Simply take Kalkwasser powder and mix into a thin paste and using a syringe, inject it into center core of aptasia and watch it melt away