Hot tub issue

I’m trying to figure out exactly what is going on in my hot tub. This is not occurring in the rest of the pool. I brush twice weekly. The seats of the gunite have started to get a little rough, what I would describe as a slighty rocky surface, when you pass your hand over it. The steps down into the pool are smooth. This is only happening in the hot tub. I can post up to date drop testing if needed. Stangely it’s just the steps. The rest of the hot tub feels fine. If I use some fine sandpaper, it disappears for a week then reappears. Any help is appreciated