It's bleaching. Not a good sign. What parameters are fine exactly, and what do you consider fine?
How about flow?
*edit* Actually, this may not be bleaching and is simply not showing polyp extension. Generally, the lower you look on the branch, the lighter it gets. This is a common thing due to the center of the colony getting less light. Lack of polyp extension doesn't necessarily mean anything is wrong, if this is in fact what we are seeing.