So, your leather sofa has decided to cultivate its own fuzzy, green ecosystem. How... artisanal. Before you start charging it rent, let's serve those unwelcome tenants an eviction notice. The provided search results gave us a lovely furniture store menu and a link to a Reddit thread, which is about as helpful as a chocolate teapot for this particular problem. So, we'll have to rely on good old-fashioned know-how.
First, if possible, drag that sofa outside into a well-ventilated area. You don't want to send mould spores flying through your house to find new real estate. Put on a mask and gloves, because inhaling mould is a party nobody wants an invitation to. Gently vacuum the visible mould spots using a vacuum with a HEPA filter to trap the nasty spores.
Next, it's time to mix your mould-slaying potion. A simple solution of equal parts cool water and white vinegar or rubbing alcohol is your best bet. Please, for the love of all that is stylish, do not use bleach or ammonia-based cleaners unless your goal is a splotchy, ruined couch. Before you go wild, test your cleaning solution on a small, hidden area to make sure your leather doesn't react poorly.
Once you've spot-tested, dampen a clean cloth with your solution, wring it out thoroughly so it's not sopping wet, and gently wipe the mouldy areas. Follow up with a second cloth dampened with just clean water to rinse off the cleaning solution. Then, the most crucial step: dry it completely with a soft, clean towel. A little indirect sunlight can help, but don't let it bake for hours.
After your sofa is bone dry, it will be quite thirsty. Apply a high-quality leather conditioner to restore its moisture and prevent it from cracking. This will also create a bit of a barrier against future fungal invasions. Finally, address the reason the mould appeared in the first place—usually a damp, humid environment. A dehumidifier might just be your sofa's new best friend.
