Selenite is a salt and will be harmed by water contact. A quick spritz and wipe might not make it crumble but we'd recommend using the light of the sun or moon, a brief earth burial, brown rice, or herbal smoke to purify your pendulum before and after use, or any other time you feel it needs a little loving care.
What do you do with a pendulum?
It's an ancient method of divining, or getting an answer to a question.