The glass should be cleaned carefully, using a lens cleaning cloth or a soft cloth moistened with a few drops of alcohol.
The glass is delicate and the reflective layer could get scratched or damaged. If the glass is not very dirty, cleaning it with a lens cleaning cloth, moistened with a few drops of alcohol should be enough. To clean more thoroughly you can apply some soapy water using a soft cloth, rinsing and draining to completely remove the soap.
When in doubt, you should clean the TeleprompterPAD glass using the same method you would use to clean a camera lens or your reading glasses.