if the bubble coral is covering the skeleton entirely and no healthy skeleton is exposed, it is ok. i believe the algae is growing on the outer skeleton and not the inner skeleton where the flesh is covering? if its just outside, its ok. but if the algae grows on the inner skeleton, it means that part of the bubble has pulled away from the skeleton and left it exposed for the algae to grow on.
if the algae only growing outside on the unfleshy part, u can scrub it or leave it alone i dun think it is a concern. but if it is growing on the skeleton that was once covered with healthy tissue, it can grow and take over the coral.