no slime, no smell and growth seems okay?
probably a stain from ferts. clay pebbles can be dusty if not washed off, but i woudln't think that would stick as well.. those are fairly large particles vs what causes staining. guess it'd be a bit gritty if from the leca balls? those particulates are "heavy."
keep the water agitated/oxygenated, and sould be fine. (bubbles stir)