We do quite a bit of glass work in pools.
Never heard of a silicone rubber grout.
More important is the thinset then the grout for adhesion. The surface must be properly prepared first, then you want to use a good white modified thinset. Then you can use a C-Cure or similar grout.

Dont know what type of glass you are using but they very greatly. Anything that is back meshed is not warrantied by the thinset company. Some of the thicker glass can also be a pain because of the weight.

You want to find someone that is VERY experianced in doing glass tile in pools. You usually pay a premium for this, but it will be worth it.