Ive had that in my E&F system before.... i "think" its from not washing the hydroton well enough... But i could be mistaken... The next time i used my ebb and flow system i went above and beyond when cleaning the hydroton and it didnt happen again...

The good news.. its just inert clay.. so it wont hurt anything.. and, depending on your pump filter, it wont even pick it back up.. which is why it tends to accumulate on the "drain" side of my res.


Good luck!