dark spot

Hey guys I have moved a large hammer coral from this space and am wanting to fill it with something. Only problem is this it’s kinda covered by this huge (basketball size) torch coral above it. Hence, when I moved the hammer coral. Anyone have any ideas as to what does well in low light?

I recently purchased a Scopas Tang from my LFS and it has been developing a dark spot just above his head at the base of his dorsal fin (see pictures). Has anyone experienced something similar to this? If so, what could be the cause? Thanks!