Whats that in my Biorb!?

So I recently cleaned out the Biorb, about 3 weeks ago and theres a brown sort of dust forming in the Biorb on the Plastic Plant! Under the water! AND there like fine hairs forming round the top of the water pump under the water again! I looks like really fine fibres making a sort of web! Any ideas!? I have just one fish in there at the moment a silver tip tetra....