One day I got a urchin and a feather duster.
I started to notice a spider web (clear / white), very long, going from these two into the filters (canister filter and power filter intakes).
It quickly turned into lota of webbing, at the time the urchin and feather duster where together.
I seperated them, but still saw webbing.
I got rid of both (the feather duster and the urchin) and never had the problem again.
Never knew what it was.................