Moss is a plant, you kill it rather than dissolving it. Once dead it can be brushed away.
To kill moss you need a specialist product, the common ones are Patio Magic and Wet & Forget. Both of them have benzalkonium chloride (also known as alkyl dimethyl benzyl ammonium chloride) as the active ingredient, and you dilute them down to 1-2% for use.
If this is a one off then get PM/W&F. If you need to do this regularly I would normally suggest buying some concentrated benzalkonium chloride, but at the moment that may be difficult as it is used in some hand sanitisers.