So, I was looking at adult work in the office today, and this old bastard who is always on it on his phone walks past and goes a pale white colour when he sees my screen. I think he saw and might want to blackmail me. Should I report him first?
In all seriousness, you didn't do anything when you caught him. Blackmail is a no no, on every level. If you use your mobile to do it during office hours, there is clearly not a moral objection on your behalf. Outing a punter to his employers because you can is not kosher. Neither is blackmail.
You didn't do anything then, so leave it now I think. If IT spots it, you didn't know what the site was. No biggie.