Spa waterfall leaking under spillway shelf

Hi all. My spa has a spillover into pool. Water is leaking under the "shelf" for the waterfall. Not a problem when the feature is turned off, as the waterline is too low to reach the gap. But when turned on some water definitely leaks down into the spa wall, which is causing some tiles on the opposite side to fall out.

What would I use to seal this gap under the shelf? Looks like someone tried to repair previously. The shelf is a ceramic like material. Any thoughts are much...

Spa waterfall leaking under spillway shelf