Water damage is already enough of a problem itself without worrying about mold too. But water damage mold occurs regularly along with the irritation of the water damage itself.
The first thing to do is inspect and treat your home for even subtle signs of water damage. Whereas dramatic water damage (such as you might get from a tub, toilet, or other plumbing problem) is quite noticeable, other water damage is subtle. Look for peeling wallpaper or faint brown spots (especially on your ceiling or the highest part of a wall). A slight bubbling in latex paint, or a shiny appearance may also be a clue. If the problem appears to be spreading, you may want to hire a contractor to examine it further (or do it yourself if you know your way around home repairs.) You’ll need to fix the troubled patch of drywall, and removing a small section may also give you insight into the root cause behind the damage. Also, check your house’s siding where the damage appears (and above, since water can flow down before it appears as visible damage).
Once you’ve identified the root cause of the water damage mold can be prevented. Wherever the water damage occurred, use a mold-killing primer on the surface before you paint. Wear a mask and latex gloves when you apply primer, as the fumes can be quite strong and the primer difficult to remove from skin.
If water damage mold has already appeared, it’s not too late. You can swab the moldy area with a diluted bleach solution (again, use gloves and a mask). You can also remove the drywall (which you may need to do anyway, depending on the degree of water damage.
Be sure to dehumidify your house to prevent future mold growth. Mold adores humidity, so don’t let conditions in your house be a mold haven.
If the mold reappears or seems particularly aggressive, get a mold testing kit to determine the type and potential toxicity of the mold. Be sure to keep an eye out for less evident types of mold (pale or pinkish molds).