The material is "クッションフロア." The apartment admittedly had a distinct smell, but I don't know if it was musty or just the smell that unfamiliar spaces sometimes have. I didn't notice it getting sharper as I got up close to the spots on the floor.
The agent said it's probably mold, but has assured me that it's likely just a stain from mold. Most everything else in the apartment seems to be in good condition and maintained well.
This apartment is only moderately comfortable in itself — but it checks one of my boxes, and it's that it has a good view of a green space and river outside.
Optional context: my other main option is a small building where the owner of the building herself lives, a quirky and friendly lady who loves foreigners. I really enjoyed her welcoming attitude, but I've been around the block enough to know that initial friendliness can turn into micromanaging later. It doesn't have a great view, but the space itself is more spacious and comfortable. On balance, I'm about equally torn between the two options.
by nomemf