The sludge may well be your average surface coating algae from the bottom of the pond. As it photosynthesises, it forms bubbles of oxygen which can make it become buoyant early in the day, surface coating algae and bits of what it is attached to, float to the surface...
A hefty rainstorm, or harsh spray of water might be effective to agitate the sludge so the bubbles of oxygen that are making the mass buoyant are dislodged and the material settles back down to the bottom.
Cutting down nutrient inputs into the lake, stepping up plant competition for those nutrients (waterlilies, iris etc.) can eliminate such algae blooms by starving the surface coating algae. Repeatedly extracting the sludge will also reduce the problem of too much fertility in a pond lacking a balanced ecology
Regards, andy
