Could you elaborate a little more? Is the issue that it won't come off at all, or it comes off, but returns, and then, what methods have you already tried?
Also, when you say between the tour-pak and backrest, do you mean along the top of the backrest where it meets the paint, or under the flap (you didn't specify which tour-pak you're using). I'd remove the backrest from the tour-pak and clean it.
Either way, should come off easily with soap, water and a brush, maybe a little cleaner wax, or even a light compound. If it's not coming off the paint and you've tried everything, sounds like maybe it's damaged the paint.
What I wonder is how it got moldy in the first place? Figure that out, and address it (i.e. cushion absorbs and holds water, causing mold, parking it under trees for extended periods of time, etc.).
Last edited by BlazingSaddle; 02-21-2017 at 03:23 PM.