1. I'm glad it's not chocolate brown. Dingy sock gray-brown is typical of many pool conditions, EXCEPT live algae, so it's not a specific indicator, like chocolate brown is.

2. Nothing wrong with pond water, if you chlorinate and have a filter than can clean it upl.

3. If it's clearing, and if DE's NOT shooting out the return, I'd recommend simply maintaining chlorine levels, and adding DE after each backwash, till the water is clear.