If it had been overworked, I would have said it's silver floating in it...
It could be just the thiosulfate going out of solution because of the lower temperatures... but I really don't know.
If it clears the film with a drop test, it should be OK.
I had the reverse problem recently. Some really overworked fixer started making a molasses colored residue at the bottom of a jug. Since it failed the drop test a long time ago, I figured it could have been algae growing in there since I mix my fixer from tap water (the same stuff grows in the sinks and toilets in the neighborhood)