I bought a flat where there had been a previous leak (which is fixed), but it left a bit of mold on the subfloor.

The floor can be sealed and there are a few sealants on the market these sealants can be quite expensive .You would usually only seal a floor where there is a continuing penetrating damp problem often in basements. You would not normally seal a floor after a normal household leak once dried out should be ok. Stephen Phillips

I would coat your sub-floor with a water based floor polish this will prevent any mould smells from coming through the flooring