It's fine to open it to add some water for humidity, but it is best to keep the lid open as minimal time as possible. Saying that, don't panic too much. Mother hens have to get off the nest to eat, drink, poop and dustbathe so it's fine for the humidity to drop for a short amount of time. I just wouldn't interfere with the actual egg from day 18. My recent chicks I hatched, one hatched a day early at day 20 so be aware they can start popping out from day 19, so keep an eye for an early baby! Good luck
