I would also agree with that. Until a tank is biologically sound there are a few strange growths. I had a search today to see if I could help you at all but couldn't come up with anything like you're describing. Having said that, I've been keeping fish tanks for over 20 years and I've never came across anything like I have just cleaned out of my shrimp tank. It could only be described as what fat looks like in a waste pipe. I have confidence it will go away as the new filter matures. I would get some fast growing cheap plants like vallis or hygropholia and bang a load in see how it pans out. Without fish you have absolutely nothing to lose. If the situation improves you van start replacing them with the plants you really want.
As said previously, the aquarium grows the problem-substance when planted and when not planted (i.e. during a fishless cycle). Whatever the conditions, the substance grows. Thus, after 6 months of trying to get rid of the substance I think it's best I move onto another aquarium with a lid. This is the aquarium I'd like: Juwel Lido 120 Aquarium + Cabinet Black ; it has the same depth and width of the aquarium (if you want to call it that) I currently have and you can upgrade the internal pump to a 1000 LPH rate pump (instead of the 600 LPH pump that comes with it as standard).
I have thrown away the following things that I've used with the problem tank: net, bucket, testing kits, pen (the pen I used to log the ammonia and nitrite levels during cycling), thermometer, heater, sand, rug (situated near the tank) and will be disinfecting the unit that holds the aquarium before I place the aquarium on it. I have a HUGE amount of anxiety when going out to feed the pond fish because of the possibility of transferring the gunk via my hands from the problem-tank to the pond.
For me, an aquarium isn't for anxiety and stress so it needs to go.