I've been trying to devise a way to reduce algae growth inside these type of steps.
Water circulation is the key here, without it, it doesn't much matter what you do, you will still get algae growth.
So, here's the best I have come up with.

Add a second return directly behind the stairs, instead of an eyeball, fasten a length of appropriately sized hose into the fitting and feed this hose inside your stairs - either located at the top to point down or at the bottom to point up, if you are using solar heat, I would recommend pointing it up since heat rises, so the warmer water would have tendencies to rise therefore circulating well from bottom to top of steps.