You say you are in R--which is wonderful. But what makes it different, this time? The things that make this a real R are what you need to tap into to get past this together, as a couple.
Even if you've got everything in place: transparency, IC, MC, remorse, empathy, etc., etc., etc., you are JUST out of the starting gate. It is not reasonable to expect you to believe that this was not an account created by your WH.
You found this account. It was not volunteered--which may mean it was not created by your husband. But it's far likelier that it was---and that he either forgot to delete it, stopped using it (and genuinely forgot it), or was hiding it from you.
And the only person who knows the answer is your husband.
He's denying. Because you're working to R, you want to believe him. That's good, really. It's good that you still retain a remnant of the Truth Bias; it gives you something to build on.
It also offers a terrific opportunity for your WH to put his money where his mouth is, step up to the plate, and move heaven and earth to prove HIS honesty--or, if necessary, accepting responsibility, giving you access to the account so you can see the communications it holds, and deleting it in your presence.
If he continues to deny that this is an account he created, I'd want a poly, given the history.
BS-me, 62; X-irrelevant; we’re D & NC. "So much for the past and present. The future is called 'perhaps,' which is the only possible thing to call the future. And the important thing is not to let that scare you." Tennessee Williams