First step is to erect it. If it has got mould you'll be able to see it. Where it is (Inner? Outer?) dictates what treatment you'll have to use. In any case, give it a good air ouyt and hose it down before you start. This will get rid of the worst of the fusty smell. With any luck though it will just be fusty, rather than actual mould.
If your groundsheet was dirty when you packed up , that could be causing the mouldy smell. Get it pitched and hosed as val says . Do not use any deteregent on the outer tent . just hose it down with plenty of water.
if it is the inner that's smelling mouldy ,that can go in the washing machine . quickwash cycle, non bio liquid , exrtra rinse , dry on washing line .
The inners in the Orchy are fixed to the groundsheet.
Are they not removable from the tent at all ? That will be more difficult to wash.
or do you mean they have their own built in groundsheet?
I have machine washed inners with built in groundsheets , as long as it will fit in your machine no problem .if its too big , wash in the bath ,or a paddling pool, but make sure its very well rinsed , get all the detergent out of it .