Yeah, there's nothing really that unusual about it. He's likely an AFC who either doesn't know or makes excuses for your ex's cheating and is "interested" in the kids bc he's sucking up to your ex (and not a dangerous pedophile). Don't let it drive you crazy.
I went through the exact same thing when I split w/ my ex - who I was still seeing daily bc of the kids - including wondering why her new admirer was so interested in the kids and telling her I thought he might be a pedophile. Shockingly, leading with accusing her of taking up with a pedophile made her less interested in listening when I came back with calmer requests (limit his interactions w/ the kids bc it can be confusing to them; see him when the kids are w/ me, etc.).
