I also agree more likely to be majano than aptasia. It is a pest anemone that you can either live with or live without.
I have heard that taking the rock out, putting it in a tupaware of tank water, and in the fridge over night, will kill the anemone. This is not something I have personally tried though.
Other than that most of the advice given for aptasia works for mojano's as well, if you do not get rid of it they will eventually spread throughout your tank and make it harder to keep other corals in your tank as the pest anemone will sting them.