I recently removed old carpet that was installed by the previous owner of the condo I just purchased.

While preparing to install Kahrs click hardwood flooring, I noticed that my wood subfloor was not level, and the wood subfloor that meets the wall that separates my bathroom and my bedroom, squeaks! The squeaking runs along that wall that separates my bathroom and bedroom.

I also noticed that there was some sprayed on white stuff along the same area where it squeaks. And there is a weird triangular dark area towards the wall that meets my balcony- unsure if there was white stuff sprayed underneath before.
The subfloor isn't necessarily soft and there aren't any strong odors.
I attached a picture for your reference. (click to zoom in)

Does this mean there is a foundational problem with my joists?

Should I hire a carpenter to inspect this subfloor for mold, water damage, or joist problems?

How much would it be to replace that piece of plywood?

Also, if I decide to just hire a professional to do my floors, would they also be to level the floor?

Originally, I planned to DIY my click floors to save money but now I'm worried there may be bigger problems, that I need to level my floor, and I am new to all this (was going to attempt installing click wood floors based on Youtube videos).

Please help!