Can’t open video but my guess as they’re susceptible to such is lymphocystis. Lympho Is viral and associated with both poor water quality and diet
Doesn’t mean your water is poor but rather does not meet husbandry requirements for this fish
Test and maintain water quality and it will fall off
Please send a couple of still pics under white lights for confirmation
MAY and I say May be an isopod or post injury.
To be safe, give the fish a freshwater dip for 4-5 minutes in tap water the same temperature as display tank and see if this dislodges. Beautiful blue angel.