I get some of those little foam beads in my ceiling lights all the time. I have no mice although mice could cause that issue.
I just remove the light lens and vacuum the foam beads.
Turn on the AC fan and sniff the air.
If you get the rig it would be a good idea to mouse proof it, seal every opening you can find.
I would wonder if this was residual material from Styrofoam packaging/ protective material used in shipment from the light distributor/manufacturer to the Heartland plant. Maybe Heartland is using Styrofoam in the roof insulation package instead of the fiberglass batting they used before. Styrofoam is preferred for refrigerator/ice chest insulation, so it may insulate better than fiberglass batt.