That would be an excellent question to get a Bill Cody answer!

In case he doesn't reply, I will give you my amateur two cents worth.

You are correct that the periphyton is at the bottom of the food chain. However, the base of the food chain is vital in every single ecosystem on the planet.

The ultimate base of any food chain is usually the autotrophs - organisms that can create their own food from inorganic sources. (This usually means photosynthesizing organisms.)

Once the fish fry in your pond deplete their yolk sacs, they immediately turn to tiny food sources in the water column. Some fry can eat the periphyton directly. Other fry eat the zooplankton and tiny invertebrates in the pond that are feeding on the nearby periphyton.

I believe even larger fish may benefit from the periphyton. The last time I was at Lake Texoma, there were small schools of threadfin shad around all of the dock barrels. They did not appear to be hiding to me, but rather feeding in the water column. (They filter feed to eat plankton.) I suspect they were eating plankton by the docks where the plankton was being fed by the periphyton.

Hopefully, someone with more knowledge will drop into your thread.

P.S. I think fighting periphyton on your white barrels in a fertile pond that gets lots of sunlight will be a difficult long-term battle. Perhaps find a stiff clean sheet of some white plastic material that you can float in the pond with the large dimension facing your viewing angle. Drop the white sheet beside the barrels first thing in the morning when you want to observe the small fish. Hopefully the fish will return to their normal behavior in a short time so you can properly observe them. Then subsequently remove the sheet before the periphyton starts colonizing that sweet surface area that they so greatly desire.