Tiling fiberglass coping and water line

Did you prove/remove the source of the staining? Otherwise it's just buying a little time until the new tiles are stained too.
 
That is not what is typically called 'coping'. That is the top of the fiberglass shell. Many installs cover that with real coping.

Not sure what you can do now. Adding a paver or concrete coping would create a step down to your patio.