The pool is:
13,500 gal gunite
Aquarite Salt/Chlorine generator
DE filter, Polaris 280
Waterfall (2~3 ft) and a bubbler on a tanning ledge. (constant aeration)
There is moss rock copping around half the pool and standard flag stone around the other half.
The pool is 3 years old (about to start the 3rd season)

My chemistry has always been pretty good (some issues this year, but I'm getting those under control)

pH 7.5 ~ 8 (constant raise in pH over the week, add muriatic to lower)
TA 100 ~ 150 (add muriatic in columns to lower)
CH 200 ~ 400 (It's been on the raise lately, still trying to figure that out)
TC/FC 1~3, CC 0
Salt 3000 (from the Goldline controller)
Just got my Taylor TK, so I never tested CYA before this season.

From the first day, I've had a constant depositing of brown silt or very fine dirt on the steps, the tanning ledge and the outs.
I've attributed this to new-pool dirt on all the moss rock, or maybe it's being blown in, but it's pretty constant. I need to sweep it off every couple of days.

Now the pool is several years going and I don't think I can blame dirt from fresh moss rock anymore. The only other source is it being blown in

-OR-

is there another reason?