my 5hp briggs horisantal from 1997 in a leafblower has that... i think it is to keep the gas from sloshing all around.
its the carb with the tank under it. the gas cap doesnt seal because when i go up steep hills it leaks out. i have no idea why.
it looks to be made as the same material as in those breather caps.
i say clean it, its inside the tank, no one sees it, as long as crap isnt going into your carb then its fine.
if you must, take it out, no big deal, it will still run fine.