Clear or yellowish residue is usually just built-up facial oils. The SoClean will oxidize/sanitize any organic residue on the mask, but an inert (and sometimes yellowish colored) material residue will be left behind. Wash the mask out well with the pre-wash or with fragrance-free dish soap. This is the most common cause of yellowing. It may seem as if the plastic is yellowing, but the yellow color may be caused by a fine film of facial oil residue.