Brass is a copper alloy, which means it is a mixture of copper (and zinc and other metals.) Because it is a mixture and not a compound, the copper can be dissolved into the tank which is BAD for corals.
The question is how much copper is leaching into the tank.
brass is an alloy of copper and zinc with the copper being between 55%-90%. this would lead me to believe that brass wouldn't be a very good thing to put in your reef tank.
Brass is made from copper and zinc I believe. Definitely not made for contact with salt water. I would get it out now before you do some real harm to your tank. The green tinge is a bad sign.
The problem is finding a PVC check valve or any type for that matter....one that's works on a 1" pvc pipe.....if anyone has any ideas where I might find one....I suppose I should replace my brass one