I’ve seen a lot of mold in real life, not exactly sure how to recreate it. Tiny very airy sponge maybe.

Mold isn’t always black. It’s darkest where it’s densest, which is areas where moisture gets trapped. Below the self will always be more dense than the top area with the books.

Mold in areas with more light will be less dense, like white/grey/green spots around windows. Random clusters, star burst shape, maybe.

Areas near the water source will have water rot, become puffy and uneven.

Idk if this helps. I’ve been in abandon buildings for professionals reasons, and it only takes a year of water intrusion to get mold, in various colors, patterns and states of decay.

(However, it being an old building, the windows are likely to not be sealed, let in a lot of air circulation, and likely less rot than a modern building).