Growth on heater

[fyrfytr]

So I've been battling nitrates this whole time. I've added cheato, added nitrate absorbing rock things. Nothing is working. I've got 0 ammonia, 0 nitrites...but my nitrates are always high (API test kit). I've got good coroline algae on my rocks. While fish watching I noticed this growth on the heater. Its got good flow back there, and if it was a video you could see it moving.

What is this, and could this be the contributing factor

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[StevenBell]

Detritus build up perhaps?Could be a sponge of some sort as well

 

[Tahoe61]

Sponge.