First things first, since you know mold is present, working in the area should be done with caution. However, I assume you have not had the mold tested to determine the exact strain.
Let me state there are a lot of variables and without seeing your situation, all I can do is make an educated guess based on past experiences.
OK, if you stop the leak, you will NOT stop the mold from growing, nor will you kill it. You will need to kill this mold that is present and prevent new mold from growing.
I think you should start from inside, however if you have mold growing under the drywall and you remove it, you will be releasing mold spores into the basement. I'm sure you already have a fair amount of spores in the basement, but adding more will only make things worse. Also, you should be concerned with cross contamination.
If your drywall is glued to the wall, scraping seems to be the only method.
Let me be frank, unless you have the proper personal protective gear and equipment, you can put your health at risk. You can also spread the mold to other areas of your house by employing improper remediation techniques.
Being in the mold remediation business, I inspect infected homes on a weekly basis and there are certain scenerios where I will advise the client on how to DIY. In your case I believe you may be in over your head.
You maybe able to work with a local remediation company to have them expose the area where you think the leak is and to apply a treatment to the entire basement to make is a safer work environment for you. Killing the spores in the air is crucial.
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