Hello! The next time you pull those steps out, do yourself a favor and get rid of ALL of the sand. Clean thoroughly. You can drill extra holes for circulation if you like. Then just allow them to fill with water, and every now and then, inject them with some chlorine water to prevent algae.
I’ve got a gross picture that shows what the inside of my steps looked like with sand, and this was when I pulled them out of my absolutely pristine Intex when I was replacing it with my current pool.
You can easily make weights to keep them from floating. Some use a couple of empty bleach bottles, filled with pea gravel and chlorine water and tightly capped, then hung from the underside of the steps.
Is your area somewhat windy? I noticed you have completely graveled the area around your pool, but the neighbor’s yard appears to be a bit sandy. You may be dealing with fine silt blowing into your pool.
Have you popped your filter open to get a look at your cartridge? That could provide some hints as well.