So I used the Ctrl+click shortcut on CSP to auto-select this layer, but it looks like there's a lot of parts that aren't part of the selection for some reason. Is there a way to fix this?

In case it wasn't clear, I wanted to sand off the parts from the layer above it that's currently clipping on this layer. I'm following Elle Kurtz's cloud brush tutorial and I don't know if there's an option to lock the layer on CSP like it does on PS. The lock layer on CSP stops it from getting altered any further, unlike in her video where it lets you paint more on to it while keeping the edges intact