Cogent TYPE-C™ HPLC columns including the Cogent Diamond Hydride™ have a very low background noise even when used with LCMS. The white film you see most likely comes from “additives” build up from your mobile phase.

Therefore is highly recommended that you use the lowest amount of additive as possible to achieve your chromatography goals. It is very unlikely you are seeing bleed from these columns since have been proven to be extremely durable and the direct silicon-carbon bonds of these phases is almost non-existent.