Thanks for the quick reply. You seem to have ruled-out some of the typical areas of concern (iron, sand, etc). But I just thought of this seemingly ridiculous question: Any chance that it's simply just dirt? May sound silly, but when I have a busy bather's load in the pool, I find it amazing how much dust/dirt gets brought into the pool from our feet, body, and even the weather. If I let my pool go 2-3 days without sweeping, I definitely have a light-brown "dusting" on the floor. Any chance this may be what you are experiencing?
Speaking of dirt, any chance that after the last sand replacement, something wasn't replaced properly allowing a small amount of sand to enter the pool from the return jets?
The only other thing I can think of would be algae-related. To confirm you would be best served to do an OCLT to confirm. Your thoughts?