Probably just silicone sealing the sub to the box.
Get us a better pic of the dustcap and we can let you know what the sub is like. Looks like an older Punch from Rockford. Even if you can't get a better picture, let us know what it says exactly.
Silicone is not the best thing to use to seal a sub to the box, and that probably means that the box was built like crap to begin with. If you have to, you can probably leave them in there, but I would probably prefer to CAREFULLY peel it off and pluck those drivers out and build a new box.