If you are talking about the little tube structures on all the surfaces .... I believe the are the homes to a fly larvae of some sort. If you kinda poke them open you can see the tiny little guys living inside them.
They are black fly larvae, they attach to surfaces and grab bit of food flowing by.
I have them as well, I think they were stowaways when I bought some pond plants.
I'm not sure if Dimilin (diflubenzuron), which destroys the hard shell of parasites like lice
really well, would work on the cases of the larva but I'll be watching for success stories because
I'm in the same boat and have been inundated for some reason this year by these things.
I've had to take my leaf baskets and used a wire brush and then placed them in a strong chlorine solution to clean them up. Let them air dry and then soak in fresh water for a few hours.
Leaf baskets are easy enough to clean out but I'm pretty sure they're not isolated to the skimmers... I've got a
ton of them in my sieve and I'm betting they're effecting my BD's pipe flow. I can clean them out but personally I'd
like to find something (if possible) to control them from coming back. I've never seen them in the ponds 10 years
existence until this year.
I think the best solution for this larvae is having a couple mosquito fish in your skimmer. it is natural and no cost. Getting the mosquito from your local mosquito disease control dept. Mine are clean because of those fish.