Please help ID this

Looks like an alga to me. If so, they likely won't be harmful unless they overgrow corals. Algae usually can be controlled by removing their nutrient source. Less feeding, better skimming, or growing and harvesting a macroalga might all be useful,. Some hand-pruning might do the trick as well.
Lobphora Variegata, I havent much luck researching controls. Possibly like Bertoni said you could try harvesting macro to compete for nutrients. Here's a link for a little more info.