They are very hardy as well. I feed my tank with oyster's eggs mix in marine snow + zeofood once a week. Few days after feeding the tank, these duncans will excrete dark murky liquid from their mouth. Probably waste and then they will fully expand for a few days and will go back to their weird routine until I feed again.
As long as they are not melting or darkening, they should be fine. Let them thrive and mature a bit before fragging.
Good luck with your healthy looking Duncan.