3 Mos after Remodel, Grout turning black-ish

We had Sicis brand glass tile installed in a pool remodel. Glass mosaic was approved for pools. Installer did dark grey bond coat (I don't know what brand) and used VERSA thinset with latex additive and with some grout in it for color (he told me in case thinset bleeds through), and Polyblend brand unsanded grout in Antique White. Less than 3 months later, the grout is turning black in some areas. It is NOT algae or mold or mildew. Pool tech tried algaecide and I tried bleach with a toothbrush. Nothing budges it. I chipped a chunk of grout out and the dark color is consistently all the way through. Custom (mfg of thinset and grout) says something seems to be leaching from underneath. The original pool remodeler is no longer in business. A different pool company says it is the bond coat leaching through for some reason. I could not find anything about this possibility online. Does anyone have any ideas or experience with this? Thank you!