It's sloppy English, and might be used a slang form, somewhere (not from where I come from, at least). Yes, "harsh" is definitely an adjective. Here's what's happened: A proper sentence would be "You're being harsh on my work." Someone has decided to (deliberately?) take "harsh" and use it as a verb. So, "to harsh [smth]" roughly means "to be harsh towards [smth]". I'll remind you here that this is not standard English, and in some respects actually sounds stupid, so please don't think it's OK to use this in your own writing. Despite that, this is one example where you see how certain mechanisms in English (eg. word order and conjugation) can be imposed on any word to create a result which is incorrect, yet still clearly understandable.

I think this is a mistake. Where did you see it? This word does not exist in English. You are right - the word harsh is an adjective.