First look for predators at night and observe your fishes too.
Also could be hydroids or something else, irritating them.
They are PEs and that looks like a type of bacterial infection to me, if there is no predation or irritation.
I would try a hydrogen peroxide dip:
10% H2O2 to 90% tank water for like 1 minute and see what happens.
Rinse colony in another container with tank water before returning it to the display tank.
Hydrogen peroxide dip once a day max, for 2 days (if needed) and then wait another day to dip it again. One dip normally should be enough.
I normally prefer Lugol's for bacterial infections, but I found out that some times that type of infection is hard to get rid of with Lugol's!!
I've lost whole colonies before with that thing happening!! Some times one of 2 polyps show the symptoms. It's in the system, or in their bodies!!!!
I think it's bacterial infection because normally there is no way to find any organism stinging the polyps. If it's a stinger it's really tiny. It looks like something is irritating the skirts, but I treat as bacterial infection with H2O2 nowadays.
Why did you take a picture with only the blue light on?
Are they under them on at all times?
You need "whites" too!
Good luck!
Grandis.