Trying to turn VFX Composited Layer into Alpha Matte, help!

Hello, for grading I'm trying to create an alpha matte for the VFX Layer, I've attached a pic of how the Composited Layer looks like in AE that was applied to the original shot, how can I turn just the red blood parts into an alpha matte? As currently it exports all the white parts as part of the matte too? It's made up of multiple of layers.