Updated now (1 day later):
Before the water change:
During water change of the 1 gal of water, with new sea water and antibiotic dosage ( 50 mg/l cloramfenicol)
After water change:
I wonder what is what I have marked in blue in the photos, knowing the polyp anatomy is:
I propose the following options:
1- Necrosis and dead tissue.
2- Some necrosis and mesenterial filaments.
3- Mesentarial filamentents and no necrosis.
Any ideas?