Bees build this sometimes. It’s called burr comb. I discovered these particular specimens in the new hives — one spreading like a pancake across the tops of the frames, the other forming a double wall between two frames.*
Here’s a close-up view:
I removed them, obviously. They’re not doing any real harm at the moment (just storing a bit of pollen), but left unchecked this stuff can cause problems for the hives later on.**
Also, I just love the look of it. Humans can’t excrete anything this exquisitely lovely, and that’s a fact.