Might be brown algea. Look up images for brown algea online and compare to your tank. If it is brown algea then its harmless and will likely disappear on its own as it is out competed by cyanobacteria and green algea.
A better photo would help a lot.
Good luck!
those are my guesses,
it looks to be on the older leaves predominantly, ...
if it's any of those, ignore it and the problem will disappear on it's own
well detritus is a maintenance thing
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