This debate is as old as cardboard time.
Some people refuse to deal with any of the 'normal' stains associated with opening packs of baseball cards - gum, wax, etc. But others (myself included) have no problem with it, because that's what they looked like when we opened the packs back in the day. So, you have to decide which side of that fence you're on.
If you try to remove that gunk, you will most likely ruin your card. There's no protection back there, just cardboard. Others will offer tips on trying to do it without damaging the card.
On a side note, that isn't glue or anything, is it? That's a whole other issue.
In general, I'd say back stains are much more acceptable than front stains to most collectors.