Its detritus--- fish poop, snail poop, food, dead things, dirt etc. Siphon or pump it out. If its too low to siphon out, turn off pump and use a powerhead with a hose on it to pump out the sump, after or while you keep the stuff in suspension, by swirling. Rinse with new water and fill to regular level. You could just do water changes by doing that. Steve.
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