say "Hey, saw you in the backyard on my security camera the other day, what were you doing"
As others said, if he has a reasonable explanation, he will respond quickly/easily with the answer.
If he struggles with an answer, you know he was up to something nefarious.
And that is the top priority, you need to know what you are dealing with.
You don't have even have to confront him at the point if you feel uncomfortable with it... But I do think you need to know what you are dealing with, so you can take future precautions and warn your family/daughter about him if it turns out he's a creeper. Keeping your family safe is your priority at this point.