Fusion Nodes turning red - how to animate nodes?

Fusion noob here. How am I meant to animate specific Fusion components, like this glossy rectangle, without making the Media-Out turn red?

I want the glossy rectangle to fade in.

The background1 only starts from frame 12. So in frames before frame 12 there is no output for the MediaOut to display hence the red color.

Doesn't explain the GaussianBlur1 failing, though.

The rectangle also starts from frame 12, so it kind of does, I think?

Yeah, but why?

This I don't know. I was just replying to why it fails