Assuming your water parameters are good, the most likely cause of the STN is insufficient flow. It's easy to say the polyps are swaying but for millepora the polyps are long so swaying is easily achieved.
Flow required is more random flow, meaning coming from different directions.Constant one directional flow does more damage than good to the SPS. The pulsing action of a controllable Tunze is the best. Clashing the flow of 2 controllable Tunzes set at different pulsing period, one longer and the other shorter, will even be better.
1.5 feet from a 250W is more than enough for millepora. So can rule out insufficient light. Anyway IME, STN occur mainly due to lack of flow and poor water parameters. Insufficient light will likely brown the coral only.