Curdled spit up is just milk that's been in the stomach a bit longer, and is already partially digested. It's no big deal, no more than non-curdled spit up would be. How curdled it is just reflects how long it's been since baby ate, and that's all. Sometimes it tastes sour, though, and smells bad, because it's already partially digested and mixed with stomach acid. That's no big deal either, if baby is growing normally and is in no distress.
If you're worried, though, or if it seems like way too much, or it just started suddenly, you could talk to a health care provider about reflux, and what's normal and not normal spit up. It can't hurt to ask, but frankly I wouldn't worry if baby is happy and growing. Two of my three were big spitters, and they're fine.