How can I fix this very noticeable brush mark?
Don't use a brush.
There is a 99.9999% chance that the original ceiling paint was done with a roller, and you should do the same.
The roller leaves those little stipple marks you see elsewhere on the ceiling, while a brush will leave straight lines. The straight lines will, as you've noticed, stand out quite nicely against the stippled background.
At this point, you're not really putting paint on the ceiling as much as you are putting a pattern on, so you should be able to do it with a reasonably dry roller. Light coats, applied in a zig-zag pattern until it disappears.