NO, grout does not have adhesive properties like a thinset would.

You need a more experienced and skilled tiler.

If he has so much excess thinset, he's probably using the wrong sized trowel for small tiles. Run away!

Well, you might need to get a qualified tile installer. He/she is not one. Move on.

Thanks everyone. Unfortunately I didn't choose him, he was sent by GC

Your GC is responsible for his sub-contractors. Ask hm/her to document how this installation meets industry standards (TCNA ) and manufacturer specifications for products used.

Let your GC he is not acceptable.

Thanks Jeff. This is just the ubiquitous 1" hex tile that you can find everywhere like from Daltile, Merola, and American Universal that replicates historic hex tile. It seems to used all the time for historic renovations so hopefully it's ok! And, yes, it says ok for floor at store (doesn't say anything on box), was purchased at Tile Shop. It is porcelain, not ceramic although I know many people don't believe in difference. Anyone else have opinion?

Please refer to our Installation Guides here - https://rocksinstock.com/pages/installation-guides-recommendations

@Kristan Milam. The usual 1" hex mosaic tile for floors is 1/4" thick and I used the version of it with a few black hexs amongst the white ones in my renovation. So I'd remeasure the thickness and if it is close to 1/4" (or 6mm) you should be ok. Then the tile person should install it in thinset using a 3/16" v- notch trowel.

so ours actually measures about 3/16" inch. And interestingly I went back and measured all the samples I have (and it's MANY) and only one was actually 1/4" (American Restoration tile I believe), all others including several that said on the box they were either 1/4" or 6mm actually were 3/16" at best, so I think mine is the same. fingers crossed! I don't think I can stomach changing tile again....

I am seeing one positive aspect here. The tile installer pointed out that he had an issue and tod the homeowner about his proposed approach. Many of the tile issues seen in postings here are situations where the install proceeded without the homeowner being consulted.  Give the tiler some credit for not plowing ahead and spending time and materials on an install that needed to be yanked out.

Looks like the tile is standard. and all that's needed is a small notch on the trowel. You should also talk about an uncoupling membrane

You have a standard mosaic thickness. No need for special measures.

NOTE - Some of the mosaics on the market nowadays are so thin (less than 1/8") that special methods must be used. Adding grout dye packs to specific setting mortars is one of those options. Merkrete has had a two in one product for several years.

THIN-SET-MORTAR-INTEGRA-COLOR-GROUT.pdf (merkrete.com)

Your installer appears to be onto something but needs to do more research for approved methods by manufacturers. Yes I do believe there are some grout manufactures that may ok their product for setting mosaics. Technology has come a long way. Approval and limitations are priority.