I live with roommates who are older and don't like weed, I have water pipe that i keep in the car and never clean it cause they'll smell it and i would get kicked out, i live in CA so the car is hot, i always forget to change/empty the water and at night i can't see the water color, so the water was a greenish color and alot of resin and ash was built up on the diffuser. Is there any long term health effects of smoking out of a really dirty bong, it was in april when i stopped using it, i never had any headaches, nausea and etc. when i took a hit at the time. Sorry for the long post.
Clean it thoroughly in alcohol before using it again. You should be fine. I would consider using a dry pipe out in the car...