When I have fished from the dams on large Kansas reservoirs, the riprap ALWAYS has growth on the rocks.

(The only exceptions I have observed are when the lake was at flood stage or massively drawn down during a drought.)

Also, there are typically some sticks caught in the rocks from past floods. In the spring, when the water levels rise, these sticks, and other stuck flotsam like litter, go back underwater and start forming growth substrates. Also there are annual weeds in some places (like sunflowers) where the stalks may go underwater in the spring.

That creates a lot of substrates for algae to form at a wide variety of densities. I suspect the GSH could find some spawning habitat among that wide range of options?

I am sure your dam is much cleaner than the dam at a public reservoir, but that is what I have observed at the big dams.