I get that light tan colored fine silt in my sump.
I figure it's fine particles of detritus that have passed through the sponge prefilters.
I'm not obsessed with this stuff - but I do clean out the sump every 6 months or so, when servicing the pumps, siphoning out the last inch of water in the sump during a water change, and wipe this fine stuff out with a sponge.
I don't know if it helps or hurts - but I doubt it hurts either way.
It's about time to do it again as matter of fact
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)