It's bubbling like you see in "bounce" mushrooms and happens in other corals. It's not "normal" but it doesn't appear to have any obvious negative effects on corals.
This study investigates potential causes of a novel blister-like syndrome in the plating coral Echinopora lamellosa. Visual inspections of this novel coral syndrome showed no obvious signs of macroparasites and the blisters themselves manifested as fluid-filled sacs on the surface of the coral...