Bumblebees are not algae eaters, they are carnivores, and good at eating left over food, dietrus, or dieing fish, coral etc.
If the food has landed on the polyps, then they may appear to be eating the polyps, but not so.
However, if the polyps are dieing, then they may be cleaning up the ones that are.
I say this because I had a small zoa cluster that only some where not adapting well in the tank, and the bumblebees were quick in clearing out the ones that were not strong, but left the ones that were. They haven't touched them since, this was easily a year ago. And the zoas are going strong.
I would trust them in a reef set-up.