Todd, The most probable cause : The water used for steaming.
The reason appears related to the heating tank coming in contact with poorly distilled water or tap water ;the water boils, minerals deposit on the inside walls of the heating tank that re-deposit onto whatever one steams in the form of a white power. Some manuals for hand held steam cleaners recommend a 50-50 blend of warm white vinegar and distilled water poured into the unpluged unit to help remove the deposits overnight. However, that hasn't worked for me.
Whenever a steamer deposits stuff or begins "spitting", its easier (for me) to buy a new one. It does seem the older the unit the greater the probabilty ,unless, one has always used super pure or greater water products. The maxium , the cleaner the water product the less likely the steamer will shower your work with unacceptable mineral deposits.
From emails I've received the issue is not wide-spread and for unexplaned reasons can occur (very rare) right out of the box. I know of two instances that were replaced by the seller without problems.
The good news is that a good brush /or de-static brush will remove the white powery stuff , as will a soft, mildly damp micro cloth.
My last suggestion is that heavly molded LPs from years in basements can have shreds of mold left behind, but from your description I think H2O is the culprit.