Looks like you may have some ich going on ... with a possibility of velvet, as well. Both can be treated with copper or Chloroquine Phosphate, but both should be treated outside your display tank. Your display should remain fishless ("fallow") for 76 days, in order to ensure that any ich theronts which hatch out are starved out.
There's a more in-depth article on dealing with ich here:
https://www.reef2reef.com/threads/ich-cryptocaryon-irritans.191226/#post-2192627
And one on velvet here:
https://www.reef2reef.com/threads/velvet-amyloodinium-ocellatum.217570/#post-2499399
There are reefers who maintain "ich management" tanks - many of them using oxydators, ozone, diatom filters, and/or UV sterilization to assist.
Some considerations when choosing between "eradication" of parasitic diseases, and "management" of the same:
https://www.reef2reef.com/threads/ich-eradication-vs-ich-management.188775/#post-2171424
It should be noted that velvet is much less amenable to management, and once it gets going, can wipe out a tank in a matter of days.
~Bruce