Doors and Windows - mold on caulking under window

I have been all over the internet trying to find an answer to my problem and so far I am at a loss. I have noticed that I have mold on the caulking that is between the window and the sill. I tried using bleach which seemed to make it lighter but not taking the black completely away. Should I take all the caulking off and re-caulk or is the faded black spots ok. I know its not good to breathe in so I just want to make sure that I am doing the right thing

First of all, mold spores are present in every breath of air we breathe. Too much hype is being given to mold and it's making too many people paranoid. Mold spores are an allergen, just like pollen is an allergen. But you don't see people cutting down trees and flower beds because of pollen. There are only a few types of mold that are really toxic, and from what I have heard, they are pretty rare. At the same time, if mold is growing inside the home it only makes sense to remove it. Kind of like how you clean the floor when it's dirty or vacuum the carpet.

When mold is first noticed, it can usually be cleaned off by using a detergent. You use detergent because it removes the mold as well as leaves surfactants behind which will temporarily prevent mold growth. Cleaning mold right away will prevent the "roots" of the mold from penetrating into the surface which they are feeding on. These are the "black spots" you observe. They are not harmful, but if they bother you, and you can't paint over it, the only way to remove them is to remove the caulking and recaulk. Sometimes an oil based caulking will repel mold better than latex. Other types of caulk specifically say they will not mold or mildew (provided they are kept clean).

It's likely that the mold is growing on that area because of cold air meeting warm air. It could be that your window does not have insulation under it, or it may not be air sealed on the exterior. In any case, mold growth is not suprising under the right conditions. Keeping things clean is an important step in preventing mold growth.

Another reason for your inability to remove the mold is your "caulking" is probably silicone based, and the mold was there prior to the installation of the silicone, and therefore grew from beneath, and won't be affected by cleaners.