I feel sure something is wrong here....never heard reports of the macro going mouldy. (this is UKAPS so mould rather than US mold
😀).
Are you sure you are not confusing micro, which can go mouldy, with macro.
Potassium nitrate is a very good preservative, thus will prevent mould and bacteria growth exceptionally well, which is why we have never heard of this before. Potassium nitrate and sodium nitrate are used as preservatives in cooked meat.
What type of water are you using, tap, boiled, RO, distilled etc. Hard tap water can cause the salts to precipitate out, so try using other water. I have 22' Clarke water (F hard...

) and use boiled water, though I have not seen any precipitation when I use tap water as someone has pinched my kettle water to make a cup of tea
🙄.
I once accidentally left a litre of macro solution on the window sill for a week, in the sunshine, after a water change, and observed no change, as would be expected from a strong solution of inorganic salts.
Did you get your EI salts from a reputable source ? Perform a nitrate test (one time a test kit can be used) to verify the nitrate is in fact a nitrate and not some other cheaper salt.