Speer Bullet packing foam

you can tumble them just like you would your brass

I've had several boxes like that over the past years, just a wipe on a rag and good to go. The target will never know their not pretty looking.

If it is just discolored spots, and not some goober, just ignore it.

Hand polish each one with a soft cloth and Flitz

You're lucky they didn't use hot spray in foam, then you would have a problem.

While not having nice shiny bullets to load would not be an issue, the presence of petroleum based foam residue would concern me because of the possibility of powder/primer contamination. Foam does not rot, it generally takes some kind of chemical reaction either airborne or direct contact to break it down if it is not exposed to sunlight. I'd run it thru my tumbler and then dispose of the media before cleaning my next batch of cases.

Foam certainly does dry out and "rot" down here in the heat it turns to dust. Depends on what kind, open cell closed cell, Styrofoam
without a picture it;s hard to tell what the OP is dealing with.

But if there are no Klingons on the bullets then what's the problem? I buy blemished bullets they have some discoloration on them, I do not polish them up.