I couldn't tell much by the video...can you post a still shot of the affected area....
I will assume this is the 55gal...correct....
How much and how often are the water changes, water temp, filtration, live plants, additives used, appetite, how long have you had her and has she ever been sick and/or treated-if so, details.....any other signs/symptoms....
As Punki recommended....I would get her in a small QT until we can figure out what is going on....if this is an open gaping wound-I would go ahead and get her started on high dose of aquarium salt 3tsp/gal, low water temp 75-76F for now......usually sudden open wounds when not injury related can be linked to Flecibacter columnaris and usually fatal within 12-24h unless it is the slow moving type and that can be treated with high doses of sodium chloride....