I think it is nothing really unless it progresses .could even be water spots from splashing.
at the end of the day it is irrelevant because the only cure for anything whether it's a deficiency or a toxicity is to flush the soil or soiless ( sphagnum based mixes) or water, with a fresh balanced complete feed solution , over watering until you have completely changed all the old solution in the medium and replaced it with new .
this is not 4ha4 but a good indicator to overflow medium more with each watering to keep nutrients fresh and balanced