It may be mold, but even if it were, you hardly need a professional to help you get rid of it, just spray down the area with bleach and wait a few minutes (not too many though, as that can damage the finish further) to let it do its thing, then (while wearing proper protection, ie rubber gloves, etc) wipe up the area using a cloth and warm water. Then continue cleaning the rest of the cabinet with the disinfectant of your choice. However, its important to keep in mind that this isn't a permanent fix.
The discoloration (brown) that you see is simple water damage, and unfortunately I don't believe there's an easy way to fix that.
The most important thing though is to identify the original source of the moisture and ensure that you keep the cabinet dry in the future to prevent additional buildup of mold and mildew.
Taken from Fred's comment:
It is mold and with fungus and was caused by a leaky faucet and/pipe (fitting).
You will need to remove that piece entirely and replace it with a new one.
Simply cleaning it will not be enough. Don't go putting a piece on top of it, because it will come back.