1) Melancholy can be either a noun or an adjective. However, melancholic can only be an adjective. Therefore, why use the same word for both when you can decide against that?
2) The suffix "-ic" is common for adjectives. Acidic, Semitic, semantic, homophobic, etc.
It's up to you, of course, but I hope that I've convinced, even if only a bit, that "melancholic" slays as an adjective. lol