Bumps similar to this were described by Brady to Gordon Parr that it was encysted juvenile anchor worm. He treated the pond with Dimilin and they disappeared. Not saying that that is what it is, and don't know the mechanics of the Dimilin in this case, as Dimilin is a chiton inhibitor preventing the development of a shell so the exoskeleton of insects and shell fish stop growing and cause the strangulation of the insect. But it is worth a try.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard