Is the wall made out of actual rock? Most non toxic sealants should work I think. It depends on the material you're sealing though. I don't think rock itself shouldn't need to be sealed.
out of foam check this video out https://m.youtube.com/watch?v=T_4gCuKRCeU I made just a basking platform it came out great I used all this products now I'm going full scale,as for grout any floor grout will do grout is basically cement that will dry so nothing to worry about it.