I agree. Without more I wouldn't call cps. An accusation of child molestation can really devastate a person's life and I'd be really careful about making such an accusation. I do think it's appropriate to tell the girl's mother and to keep an eye out if you're worried and certainly to call cps if you have a real reason to believe she is being molested.
There is a little girl at my son's school who I hardly have any relationship with at all, but whenever she sees me she'll run over and throw her arms around me. And if I'm standing still, like talking to someone, she'll lean her head on my side and just stay there. It really kinda bothers me and definitely tramples all over my boundaries but I feel powerless to get out of it. I don't want to hurt her feelings bc I suspect she's not getting a lot of motherly affection at home. So unless you saw him being physically aggressive or manipulative with her, I'd say you don't really know how that came to pass, kwim?