Green blood!
Non-divers might not know this, but the deeper you are in water, the less red you see. While this typically means not being able to spot red fish in the water, it can also mean that any cuts you have will look like they’re infected.
When this person scraped their arm on a wall of coral, they understandably thought that something had gone completely wrong. The cut looked blackish-greenish in the water, and they thought that maybe he was cut by a toxic coral. While some coral can be toxic, this wasn’t the case in this story. Thank goodness for that!