Should blue-gray marks all over your spa and pool plaster be coverd under warranty on a 3 month old pool?

If it is the plaster discoloring then the plaster company should fix it. If you let your chemicals get out of balance it is your own fault.

The blue/gray marks can be from your leaf skimmer - it has an aluminum frame- so when you scoop it leaves marks especially if you are using it with the net inverted. But those gray marks are only about as wide as a pencil and close to a straignt line. Broad marks indicate poor chemical maintenance. Your problem not the builder.