Rough one. I would guess it's just porn he watches and anything with his kids would not happen but since you don't know, agree telling his x wife what you found would be best to make sure they are safe and then you just keep moving and don't look back. This is not the guy for you.
Tell his ex-wife and ask if she is concerned about his intentions towards his daughters. This is about their safety, even more than about your nausea.