What do you have your water returning to your tank running through? ie ribbed hose, clear vinyl, pvc? I've seen this on ribbed hoses usually and only ribbed its a build up of bacteria. Its dead and it falls off hose wall when power is cut then when pump is turned back on it gets pushed into tank. I started having same issue with my hoses on my fluval canister filters. I didn't know what was going on, lfs said to use clear vinyl and it stopped happening. The bacteria colonizes the inside of the ribs and builds up and then dies. No ribs nowhere to colonize.