Thanks for all the advice.
We are draining it right now, as I write. The guy at the pool store thought it looked and sounded like paint - that's possible, but we'll never know. Anyway, I will water down the bleach when we clean it.

We're not ready to get a really expensive security system for an Intex that is only up two months a year. However, this did get us to thinking about improving our security and we may eventually get a camera system. We are thinking of putting in the motion light and an alarm that goes off if motion is sensed - but we realize it will probably go off when animals go by etc. But we need to do something in case the clown who did this tries again.
Barb