If it hasn't gone away in 3 weeks I'm afraid you're wasting your time trying to get it to dissolve. If it was going to, it would have by now.
If it's fine enough to go through a sand filter I'd put a sock in the skimmer and see if it'd catch it.
If it's fine enough to go through a sand filter I'd put a sock in the skimmer and see if it'd catch it.