The stains may be a combination of water stain and a sort of dye made by the mold. There are three different kinds of wood bleach and they each act on different stains. For all these bleaches you will need to remove all the finish from the wood surface, either by sanding or chemical stripping. If you sand this will give you a chance to see how deep the stain is. If you are really lucky you can sand most or all of the stain out. Use 80 grit sandpaper, and finish with 100-120 grit.
The first is oxalic acid, in liquid or crystal form. This will remove most water and rust stains. Apply a strong solution and wait several hours or overnight if needed. Wash off wood surface with lots of water, with a bit of baking soda to neutralize the acid. A lot of deck brighteners have oxalic acid in them.
The next to try is a chlorine bleach. This will remove most dye stains, caused by juice, blood or coffee spills. The weakest form of this bleach is regular household bleach. It's worth a try ,but it may be ineffective. Better to use swimming pool bleach (dry calcium hypochlorite), and mix with hot water, until solution is saturated. Apply to the spot and it should work right away, but try twice if needed and wait overnight. Wash off with lots of distilled water and let the wood dry overnight, before refinishing if it did work.
And lastly for organic stains, like urine and feces, your only hope is to use hydrogen peroxide, in a very strong and dangerous concentration. Klean-Srtip is readily available in North America, and comes with good instructions. Visit their web site at www.kleanstrip.com for info on how to use this stuff. Click on the product button, them the wood bleach link. This will generally turn the dark spots to a off white color, which you will have to blend in with a light colored wood stain to match the color of your finished wood floor.
With all these bleaches, they are water based, so they will raise the grain and make the wood rough. Be sure to let the wood dry overnight. And be careful when you sand it smooth not to sand out the bleached effect. Final sand with little pressure with 120 grit sandpaper.
Or you can repair the spot. Read the wood floor repair article available in the search box at the top of this web page.