The car may have been parked up for a long time with the roof having been folded down while damp, hence the mould growth. Clean it off as suggested and there are some proprietary mould and mildew removers available in your local supermarket. Treat a small area first to make sure it's safe, then do the rest. Might be no harm to check the drainage holes and rain rail on the hood. Lift the parcel shelf carpet , and may be no harm to check under the floor carpet as well.
A good airing is always helpful so even if you have a hard top for Winter, give the car a day or two with soft top up every now and again just to blow off the dampness.
If there are signs of mould or dampness under the carpets then you need to tackle that as well. Let us know how you get on and someone is bound to steer you in the right direction
John