Homax Popcorn Ceiling Texture Spray

The crack may be caused by the owner failing to tape and patch the joint properly, and simply spraying over it. If that is the case, just putting something on the crack may be only a temporary fix before it opens up again due to seasonal movement. You can try to just patch it now, but, if the crack reappears, you would then need to decide whether it was troublesome enough to actually scrape the ceiling (at least in that location) to tape the joint better.

If movement of the framing is the cause of the crack, covering it with a molding or fake beam would be a solution.

What should I patch it with?

Your best option is to do what kudzu9 suggests: scrape and patch correctly. The chances of caulk or spray paint holding up for more than a short time are, imho, pretty remote. Scrape off the popcorn, use wide self-adhesive fiberglass tape - the newer thinner type is especially good for this kind of repair - coat with joint compound, sand and repaint. This will be a tedious process for a long crack on a high ceiling, but once done it you should not have to do it again.

If by chance you find that the crack has appeared through a previous taping job, it would be a good idea to have an experienced builder take a look to make sure there is not some serious movement of ceiling joists involved.