Help: Clumping Sand Bed

[RuGlu6]

Could be H2S
Hydrogen sulfide. If you say it turns gray take a clamp and smell it if you can small rotten egg then it is h2s.
May be combination of factors like over feeding, lack of or inadequate water flow, graine size is small therefore packed, no worms to clean up the sand etc.
One more reason to go BB (Bare Bottom).