Is there a way to make a prim not cast a shadow? Basically it would have alpha properties but be visible. I'm just trying to use a prim as a green screen for photoshop and not have it interact with the environment.
Alpha blending/masking only determines how transparency is treated.
Alpha blending allows partially transparent pixels.
Alpha masking allows only fully transparent pixels, or solid pixels.
I assume your green screen is a completely solid surface, so it wouldn't have transparent parts to begin with. Changing its alpha-mode in that case should cause no changes to its behavior -- especially not the shadows it casts (or are cast onto it).