It MAY just be the paint. Pick away at the paint and open up a small area so you can clearly see the metal below. If it's actually the metal that is cracked then you need to evaluate and monitor the crack to see if it continues to grow. But this does not make the frame unrideable. At least not at the moment. The crack, if it is actually into the metal, is on a part of the frame that is pretty much 100% cosmetic. I'd be inclined to drill 1/16 or 3/32 holes through the ends and in line with the lay of the crack and from there monitor to ensure that the crack does not spread from the present size. If it does then deal with the issue. If it stays politely idle then you're in no danger of a sudden frame dissassembly while riding.