A wet vac will remove it, but you need to keep the inside of the car dry otherwise it will just come back in the future. I used to get a small tub of silica gel and leave it in the car. Seemed to work.
I use some moisture catcher things that are only £1 from JTF and only need replacing every month when the crystals have turned liquidy. Makes it smell fresher too.
Just use leather cleaner and a leather sealer to form a barrier and as stated above leave the windows open a little … seems strange tho it happening in a garage unless your garage is damp ?
I had the same issue as you. I cleaned the seats with a foaming upholstery cleaner a couple of times and can highly recommend a cat litter tray filled with Sainsbury’s or Tesco clumping cat litter. Despite my car being in storage with the windows shut, last time I looked there was no mould back on the seats anywhere!!
Can you possibly bring the seats into the house for a few weeks and allow them to dry out naturally in the drier atmosphere. Don’t put near a heater or try to dry with any artificial means.
Once they’ve thoroughly dried out then attempt to remove the mould but start gently perhaps using a vac with an upholstery attachment then sponging with upholstery shampoo suitable for your upholstery.
Keeping them dry and aired when they’re back in the car is more of a challenge.
Best thing if possible is to get them indoors if the car is going to be sitting out (even in a garage) for a long time. I had my seats up my loft for several years, and they kept fine up there. Came out as nice as they went in. As for the clean up, depends how bad it is. My back seat was a bit mouldy before I put them up the loft, but I found that warm water and a mild detergent got rid of it, just took a good few goes of scrubbing at it with a cloth (not anything abrasive of course!)
My car is now in my garage for the winter, which isn’t ideal as the garage leaks a bit. I have however found that by keeping the car under a halfords breathable cover, it is remaining almost completely dry. On occasion when it has been very cold and raining heavily, there has been some condensation on the car, however once the rain lets up within a day or so it is all dry again. The breathable cover does seem to do it’s job. I have left the garage window open a bit to allow draughts to circulate, and this definitely aids in letting the breathable cover do it’s thing once the rain stops.