Very common on bluish tangs. They like to cozy up to and wedge themselves into the rocks at night. That being said, keep an eye on it so it doesn't evolve into a bacterial infection. Do you have any antibiotics and a QT setup if needed? If an infection does develop, time is your enemy.
Here's Humblefish's bacterial infection treatment advisory:
Bacterial infections:
Symptoms - Sometimes it's self-describing: Popeye/cloudy eyes, fin & tail rot, dropsy (bloated fish), etc. But some other bacterial infections, such as furunculosis, hemorrhagic septicemia, etc., can have varying visible symptoms. Any redness or open sores/wounds on a fish should be viewed with suspicion. Also, a white film or fungus-looking growth may denote a bacterial infection. It’s important to note that in many cases a bacterial infection is usually secondary in nature to a parasitic infestation such as ich, meaning if a fish has been battling ich for a while then his immune system has been lowered. This makes infection more likely for many opportunistic, harmful bacteria.
Treatment options - Broad spectrum antibiotic medication i.e. one that treats both gram-positive and gram-negative bacterial diseases. Examples of this include Furan-2, Kanamycin aka Seachem Kanaplex, Nitrofuracin Green Powder, Triple Sulfa Powder, or a combination of using both Erythromycin & Minocycline. The latter can be accomplished (albeit expensively) by using freshwater Maracyn 1 & 2 and then doubling the dosage for saltwater use. When battling a particularly nasty bacterial infection, combining Furan-2, Kanaplex and metronidazole (ex. Seachem MetroPlex) can be very effective (and safe).