Grout/ mortar for spa waterfall

[peril]

Hey all, I noticed my waterfall wasn’t working well at low flowrates and the mortar is eroded from the edges of the waterfall. Am I ok to just use premixed grout that is sand colored from the tile aisle at Home Depot, or should I use a mortar ?

This seems simple enough, any recommendations?

Photos attached

 

[kimkats]

I am going to ping someone who may know: @1poolman1 @AQUA~HOLICS

 

[AQUA~HOLICS]

Type N mortar, resist cracking from temperature expansion and protects from moisture penetration.