The epoxy has settled under the tile due to voids within the setting material. Always obtain a full coverage of setting material around edges of tile during installation if grouting with epoxy grout.
This would generally result from a soft sponge during clean up, not using enough water during clean up and not filling the joints properly during installation.
Air was trapped in grout mix during application; excessive water and agitation used during clean up; soft float used instead of a hard rubber float for installation of epoxy; lack of full coverage of setting material under tiles.
Improper proportioning of grout ingredients; grouting when there is standing water in grout joints; grouting when temperature is below 65 degrees during and after installation; mixing other non-approved products in the EpoxyPlus 2002. TexRite EpoxyPlus 2002 is packaged with the exact amounts of all ingredients to be mixed in their entirety for a particular grouting application.
The EpoxyPlus 2002 grouting material was improperly mixed; setting material exposed in the joints; cleaning with contaminated water/ water buckets; using cleaning residue to fill voids in original application of grout; all of these can contribute to discolored epoxy grout.
If the installation is 24 hours old or less, try a mild, dish soap and a white scrub pad. If the installation is less than 72 hours old, use C-Cure Citrus Cleaner with a white nylon scrub pad. If the installation is 7 days or older, an epoxy haze remover type chemical will be needed.