Duncans can not be evaluated like most corals based on polyp shape and extension.
to evaluate your Duncan use these two modes:
1. Is any tissue along the calice of the coral pulling up / declining like receding gums and revealing white skeletal material along the polyp? If not, disregard what the Duncan is doing currently it’ll decide to be happy in a week typically.
2. evaluate all other corals
if well, and no recession at base on Duncan, it’s just doing what they do.
if it’s color gets any lighter then direct feed it more and or move it down out of bright white lighting, this can irritate for sure but if other corals seem ok with it this one will adapt too. The light is very white and not blue above, if my kessil gets turned to the 10K setting my brain coral will bleach in a month but set to total blue, perfection.