I've reduced the gunk with rubbing alcohol, but don't
need to entirely remove the stuff from the label side,
and let it be dirty looking. I've bought a cheap opera
set on DG from a store a couple years ago and one of the
CDs was piled with the foam-goo. I lightly rubbed it with
rubbing alcohol, which actually removed most of the
printing from the label side of the CD, leaving it blank
looking, yet it plays fine.
I remove those awful foam pieces everywhere I find it.