Yep, checked to see how much chemical we had left in the jug today, and the grey particles got mixed in, So just cleaned it up my with a coffee filter, and then tossed the grey filter in the trash. The grey particles tend to be 'heavy' and collect at the bottom of the jar or what ever your storing the D76 in. Therefore we tend not shake the bottle up or drain the last bit of it out for use..
Have no idea what they are or what causes them, but it has never been a problem for me.