I didnt see there was po4 media in play, well done catch team. Yes on the regenerating system we power clean it (any partial work is a recycle risk, but tap level work is no recycle risk, irony, in thread) and then we run nothing beyond rocks water and sand, no dosing for items thats all algae control and to have corals retracting in the presence of po4 stripping isn't rare.
you'd feed the algae targeted for the next two mos, adding mass, and doing more weekly water changes vs media filtration to reduce work. replace the cruise control w feeding and export and those corals will mass back up for sure.
the sandbed part is likely not a culprit knowing details so far, but its still exporting vs retaining action, whats in the bed eventually fuels a cyano challenge after you beat this one
it levels the field in terms of irritants, we run it on perfectly working tanks as prevention or when they move homes and they continue on fine, so its simply a resetter action. whether or not it fixes it varies, but the process itself is factually not harmful as long as no cloud is set up in the new tank. if we can reach in a swish rocks and get a cloud, then thats a cloud included.
if the new sandbed can be reached and grabbed and not cloud, then its eliminated as a factor
*agreed on carbon call above thats always good as backup.