This might be a Blue Throat Blenny but color on the throat is not exact. All Blenny are very territorial. This Blenny thinks the reflection in the mirror is a trespasser trying to take his hiding place. Just like a male, puffs out the chest, spreads out the sail fin and goes nose to nose. Fairly small fish (notice the fingernail). Shot with 100 mm macro Canon lens and a 20D camera (click on picture for larger version).