This comment is so funny I had to check: The ingredients seem kosher BUT they never got Kosher certification so it's technically true.
And when I looked into similar packaged drinks, I found that almost none are kosher. Some of the same drinks ARE kosher when dispensed from a machine, though, so maybe it's a preservatives thing?
Some very solid evidence that Slimer was not Jewish would be his glizzy gobbling. He didn't ask the vendor if it was kosher, he just ate em.