If you do seal it, only seal the spa side. Better yet, have it redone and use a high quality hydraulic cement or epoxy grout. From your pictures it looks like nothing more than Type S mortar was used which is not the correct product for that application.
I am not a fan of using sealers for waterproofing. If the spa wall was built as you describe, then hydraulic cement would be a good fix.
I am not a fan of using sealers for waterproofing. If the spa wall was built as you describe, then hydraulic cement would be a good fix.