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This is probably not what you have in mind at all, but the red channel makes a good basis for the moon mask and the blue channel will do for the rocks. If you are not familiar with the process of converting channels into layer masks, just say so and when I'm a little less brain dead I'll try to explain it. Or someone else can jump in.
Or, as Mr. Mouse pointed out, you can easily just paint the masks using a brush since the shapes are not complex.
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