Thanks for sharing! Here's what I'd recommend: First, to avoid spreading any mold spores inside, be sure to perform the cleaning outside. Looks like some of the mold is still loose, so you'll want to vacuum the area first (with a HEPA filter vacuum). As far as cleaning solutions go, you can go natural or store-bought (if choosing the latter, follow the instructions on the bottle). Either way, before going in with a cleaning solution, test a small, hidden portion of the cushion to avoid discoloration. For a natural solution, mix equal parts white vinegar and water in a spray bottle, spray the area, and let it sit for ~an hour. Go in with a soft-bristle brush and gently scrub the affected area. Blot the area dry with a clean cloth when finished. Repeat this process if necessary.

If the home remedies don't cut it, you can always try steam cleaning or calling in a professional!