Soft scrub or grout that wasn't completely cleaned during post construction cleaning. It may be a good thing to recommend to custy a good clear semi gloss enamel heat lamp dried will protect and beautify/make it pop!!
A palm sander should work. May get hot and need a rest after some time though. You putting a shine on it?. If yes MPs should work if it's not too bad. Even if you leave a few scratches, it won't be too noticeable in a shower. Bring some wet-dry sandpaper to take out deeper etching if it bothers you (400 , 600 grit).
Yes I do Bob. But in the other side of my business the works toilet bowl cleaner is used almost daily in tile and stone showers with no problem. We do not and I dont advise letting it have any dwell time as we apply and remove.
The "works" is 20% acid so its very easy to see what can happen if you dont use it fast.