How to put a texture over everything in materials?

I have stars as the background texture and I want to add a moon on it. Right now I’m using the “add” function in materials. But it makes the stars appear over my moon. Is there any function like “add”, that overwrites everything A has and puts B over it?

A lerp with a proper mask.
you can probably derive the mask from oversaturating the moon texture. Since it’s probably already bright.

When this happens I copy and paste the thing, then it becomes top overlapping thing - by thing i mean BSP or mesh