I am not an expert, but I would ask the supplier for cleaning recommendations first since they admitted they have heard of the problem. I would start with the most gentle method first - maybe just simple soap, then maybe alcohol, acetone, then mineral spirits. I wonder if a magic eraser would work? Regardless of what you try, I would try cleaning the marks off some pieces that haven't been installed yet, just in case.

Hopefully someone with more experience will chime in.

Best of luck to you!!

I could be that once the thinset is totally dry the pattern will disappear.

I used marble tiles in a mesh mosaic and had those marks. I didn't notice them right away. But I washed mine with dish soap and rinsed them, the whole panels before installation, once I saw what was going on. I noticed that when my tiles were wet the pattern showed even after washing, but once dried it disappeared. I think the moist marble tiles becomes translucent, yet the glued areas resist the moisture more, so there is a dryer grid work that shows up. Just a guess.

Finished mosiac that often showed grid marks when wet (not sure if this section did or not but probably, I just had to start ignoring it :)

kris
Can you send a picture.
Just making sure these mosaics haven't been set yet?
They came out of the box that way.
You may have to use a light abrasive.
At production they probably box this product partially wet.
So like you said(white marbles are very porous)the material of the mesh transferred. I think some light honing will take care of this.

This happened to our entire lot of marble tile for the foyer and hallway. We had to send them back to the supplier (Paris Ceramics), they honed them and the marks came off.

I'd try a red Scotch-Brite pad in an inconspicuous area.

Late update: I believe the markings are gone, well at least I don't notice them. We scrubbed them using a 3m pad and mineral spirits and rinsed well. I am considering purchasing 3 x 6 marble for wainscot, but am reluctant based on my experiences.

I'm writing this for any future issues with this mesh. Mineral spirits can leave an oil on the honed marble. I had the same problem with mesh markings. My marble (countertop) installer said to use Lacquer Thinner instead. They clean honed slabs all the time with lacquer thinner. This did take most of the mesh markings off my honed tile. I then took #0000 steel wool and worked on any remaining markings. Even this fine steel wool can hone the marble just slightly more, but honestly I can't tell between those where I used steel wool and those where I didn't. You might just experiment. So far I haven't seen where the lacquer thinner has dulled or lightened the marble tile, but my countertop installer said I could use an enhanced penetrating sealer versus just a plain sealer when we seal the tile. The enhanced sealer will darken the tile (but won't change the finish from honed to polished) so experiment first.

Ventilate well and turn off all pilot lights when using flammable solvents like lacquer thinner.

FYI - We had a similar issue. 2x2 hex has not been installed, but I think the mesh marks are actually glue residue. I was able to erase them with a pencil eraser off of one sheet. Tiler said soap and water may also take them off. I will keep you posted.