As your post is in the T5/T6 area, we’ll assume it is a T5 /6:
the roof lining should be available via a VW dealer.
It isn’t going to be cheap, I know that the headliner for the “living area“ directly under the bed is about £600.
removal and refitting may be more of an issue due to the highly restricted space whilst the bed id in position, I suspect the bed would need to be removed, to allow access, which can be done.
My T6 bed was removed and retrofitted with a T6.1 bed, so i am certain the bed can be removed relatively easily With a little DIY knowledge.
removal of the upper liner may be more challenging than the lower one, but I’m sure it can be done (I just don’t know how).
consider you may have the pop top lights which will need to be disconnected, along with the hangers for the safety device (net)
My question would be : why has the liner gone mouldy in the first place, is there a leak, particularly as you say the board looks black ?
Has something been spilled / sprayed onto it to cause the mould?
was the roof put away when wet ?
is your van still in warranty?
have you spoken to a VW dealer about the issue ?
if you can provide some more info, I’m sure you will receive some more targeted responses.