AIO that the guy I’m speaking to still follows a girl he used to be intimate with

For context, me and this guy have been seeing eachother for about 5 months. He got out of a long term relationship in June last year and after that he was seeing this girl casually for a few weeks at the end of July/early August, not long before we met. They stopped seeing eachother because he only wanted strictly casual, and she was leaning more towards serious. He still follows her on Instagram and interacts with her content. She posts quite revealing pictures on her posts that include her just in her bra and underwear along with normal photos. Am I overreacting or do you think there’s more to this? I’m not sure if they just recently followed eachother back again or not. Do you think I should mention it and if so how would I go about doing this without sounding too accusatory?