i have white mold on top of my rooting cubes sometimes when the cube stays wet and there's light hitting it. scrubbing it off, covering it or burying it in the soil when i transplant fixes that. it looks like little white balls. and if you try and pop them you can see the spores fly.
if it's furry like really thin cotton then that's fungus i believe. can be badd shit.
but if it looks like truepunks when it's dry then it's definitely leftover dried salt and is not a problem.