Will coughed up a jelly slug.
Notice the seeds along the side of the jelly slug… I’m thinking it was a blackberry jelly. Black because the Upside Down is dark and seeds because… you know… it’s a berry.
“Slug Puke” is probably going to be one of the new powers that Will has now after being infected by the jelly slugs during his time in the Upside Down… and it will ultimately help the boys rescue 11 from the Upside Down. So that’ll be fun.