Looks like mould. Good thing is it is on the subfloor. Pull up the damaged areas, inspect the PLUMBING (yes...pay someone) and put down new sheathing.

The interesting part is the 'white paint spray' is over top of some of the black stuff. This tells me the subfloor was exposed DURING the painting process. It is entirely possible the previous owners KNEW about the mould. If they did, they are possibly on the hook for the sheathing. The bathroom stuff is probably YOUR issue. If the old homeowner couldn't see the issues they could not declare it and are therefore not responsible for something they did NOT know about.

I don't know where you are nor how long ago your purchased but you can reach out to your realtor to find out the real estate laws in your province/state/country.

But the short answer is: Yes. This is mould. The issue is not the mould itself. It is the WARNING it is giving you about OTHER THINGS that is the most disturbing thing. Let me guess, the small mould puddles are on the other side of the bathroom wall aren't they?

Thanks so much for your reply. There actually no mold on the other side of the wall surprisingly. We’re removing all the subfloors and replacing them all.

I don't think it's mold at all....just water stains.

I'd dry it out and if it wasn't spongy, I'd paint it with Kilz and move on.