When we had our Bubble coral, we just fed it small or diced pieces of any raw seafood - scallops, oysters, krill, etc. If you use larger seafood (scallops, etc.), about a dime-sized piece is good.
Just *gently* tuck into it bubbles. It should stick to it/grasp it. Nightime is a more natural feeding time for these corals.
You'll be surprised at how much, and how quickly, it'll consume the food.