How to fix a vanity mold built up as a result of valve leak?
If the particle board is stable I would suggest spraying the affected areas down with hydrogen peroxide.
The hydrogen peroxide will kill mold and not smell as bad as using bleach. (Bleach will also kill the Mold but may cause more damage than peroxide.)
After you are sure the mold is gone if the wood is stable it should be ok. Left untreated this could cause a health risk.