Sea Ray

I had the same issue. I ended up sanding it down with fine sandpaper and buying "Sea Ray" white paint and touch up painted the affected areas. It turned out so well, I painted almost all the seats

Careful with the Bleach. It will destroy your stitching and seams will start opening up. Many have had very good results with OxyClean. Create a pasty type slur, work it in with brush and rinse WELL. There are other commercial cleaners such as Starbrite Mold and Mildew remover, again - be careful using anything with bleach. If you do, rinse it thoroughly with water and then rinse it again.

I just bought a 290 Amberjack and the seats looked like they were never cleaned. I tried all of the above suggestions plus scrubbing bubbles, spray 9, simple green. What finally worked was a product call Honey, sold at Advanced Auto. I put it on, let it sit 5 min and used magic eraser. Brought the seats back to original beautiful. I followed it up with the Honey conditioner. look like new

X14 mildew & mold spray or LYP Cleaner saw it on LI boating club video with Eddie.

I've used "Marina31" products for years. Have been extremely satisfied with there products performance.

Try using a teaspoon of oil or cloves to 1litre of water . The mix will kill the mould and it should just wipe away after 12-24 hours

Supposed to read oil of cloves

Sounds like you did not have enough ventilation in the wrap cover... There are many hacks for cleaning but, from my first hand experience, depending how "set" the mold if the mold is, if color is very light/white, you will end up having to repaint. Assuming you want the cushion to look presentable.
The Magic Erasures work wonders but require a lot of scrubbing; painting them will restore to like new in appearance.

Since my boat sits in the water year round on the gulf coast, I fought this issue for several years. I tried most everything in terms of chemicals. I now use Marine31 mildew remover gel. Spray it on, leave it for 5 min and if really bad, hit it with a brush. Then just rinse off with water. Then coat it with their mildew guard. I do this process every six months and my seats are 10 yrs old and look almost new. I should be getting a finders fee but have used various waxes, polishes, cleaners etc and over time have switched to each of Marine31 products and they have all outperformed their competition. I used to use Maguire's cleaner wax and polish. I now use Marine31 Captains one step compound and polish and instead of going over the boat 3 times, I now do it once and get a superior job.

I too use the Marine31 products. I ordered Boat Oxidation removal Kit which at the time also included a really good book on process, product, equipment, technique (Mike Phillip’s How to Detail a Boat with Marine31). I used it to select a Dewalt DWP849X buffer, and there are some kits from suppliers that include the buffer. My first time use was a 3-step process to get ride of heavy oxidation, but now I generally use just the 2-step process with polish then wax. My 1998 boat hull looks like new!

I use starbrite jel in a spray bottle and it did a great job