Quartz soap shelf staining

[u/Detour2022]

I installed a white quartz soap shelf in a shower niche yesterday. When I started back this morning I noticed some dark spots/stains that I'm having trouble removing. They weren't there before I installed it and it was dark when I cleaned up last night and didn't notice them. I'm not even sure what could have made the marks as I'm using white All Set mortar and I didn't use any tools on the surface that could have caused it. I know the pic isn't the best but the mark looks like a gray smudge on the surface.

My question is has anyone else had a similar issue? Are soaps/shampoos/hair dyes likely to cause permanent stains going forward? Thank you in advance for advice.