for every Web Site SSL Scanner creates a certificate this certificate must be stored to the disk. Usually I would not expect so many certificates placed into the folder. MWG creates the certificate before connecting to the remote server, so maybe some tool/scanner has tried to connect to many not-existing domains, causing the SSL Scanner to write so many certificates.
It should be safe to stop MWG, remove the certificates and restart MWG again, which should clean the disk space and allow MWG to continue running. SSL Scanner will create new certificates. For detailled instructions on how to remove the certificates I recommend to talk to technical support to obtain the supported procedure. I personally would simply delete them, however this may not be the official procedure.