Hi Rober, I agree with it being wax moth residue. If it were me I wouldn't through out any of the frames or foundation. It's still good to use. However.. I would strongly recommend putting those frames and foundation in a freezer for 48 hours minimum. Then put in storage.
As far as getting the bees to draw wax on the plastic foundation.. definitely frustrating! Here is what I do. I start with 2 frames/foundation. If i have a very strong hive stuffed with bees, I put blank frames in the honey supers close to there the bees are filling drawn comb with nectar. My goal being getting the wax 'started' on the frames. It's hard for the bees to get the wax started on plastic, it takes time. But when the bees need wax quickly during the flow, they really get it done much quicker. Even if they get half of a foundation started with wax, at least that is something. So the bees will be drawing wax to store honey mostly, but the two frames that I put into the super, I just rotate out as soon as there is some wax started, like half of a foundations worth. Once they have wax placed and started, they can really finish it later when they need it.