Strange deposit on fixer tray

i recently starting using photographers formulary T5 fixer and i tray develop my 4x5 film. Everything is fine with it, but i left the fix in the tray for about 2 days and there is now a coarse deposit, like salt was glued to the bottom of the tray and its extremely difficult to remove! the photo i have attached shows my fingernails trying to scratch through it. This is NOT your usual fixer stains. In the decades i have been a darkroom photographer i have never seen this before. Any ideas?

The specs on this fixer sound very good. I may switch to it myself.

https://www.digitaltruth.com/products/product.php?brand=photoformulary&link=tf5&code=03-0200

As for the stains, I've never see anything like you describe. Calling them would be a good idea, maybe they know something. When I've called them in the past they were very helpful.

hmmm... used tf5 for years and always left it in a tray sealed with another tray floating on top, 3-4 days..... never a problem.

Ive used tf4 for years, if left in open trays for awhile, it will start to form “crystals” or “salt deposits”, really not sure what they are. After about a week in the open tray most is crystalized with very little liquid left, hot water cleans it up. Tf4 is by far my favorite fixer.