I’ve had one friend with mold everywhere on her waterpik. They don’t talk about it enough, but when I opened up the bottom of the pik, I did see green mold. And also in the area above and around.
The recommendation is to use vinegar, but bleach seems much stronger. Also eventually the interior of the thing clogs. Sigh, they should make the things easier to fix.
In any case, mine is three years old and I guess they do have a life 🙂 Make sure to keep your clean.