water's washing over a lot of exposed mortar. Could be efflorescence washing down the wall tile face, or like cpaul1 said it could be calcium (if you have hard water). I'd take this picture to a pool tile supplier and see what cleaner they recommend. Given that it's a ceramic tile, anything super abrasive should be a last step.

It's not calcium because this has been happening since it was first built 18 months ago.

Contact the pool builder and see what they say about it.

Salts leaking out the natural stone ?

if it was almost immediate, then my money is on the mortar. I'm seeing mortar joints that are as big as my forearm, that's a lot of surface area for water to wash across and extract funk.

if you live in a climate where it gets below freezing in the winter, make sure that part of spring startup for the pool is inspecting all the mortar joints and repointing as needed. You don't want small rocks or chunks of mortar in your pool.

Very nice water feature. You have natural stone held together with mortar. The water is allowing the minerals in both to leach out. Try purchasing a good sealer from a stone center. This will help stop the issue from reoccurring. You will need to clean area well before you seal it. Try a mix of water and muriatic acid (3-1 ratio) 3 parts water 1 part acid. The acid may affect your PH but won't hurt the pool. Once clean allow area to dry and then seal it. This will prevent the water from penetrating the mortar and stone. Use a good sealer that penatrates into the stone and mortar. Hopes this helps.