Help! Something attached to sea hare's face

The tank is a 55 gallon and its 4 months old, we were told to leave him alone because he eats algae off the rocks,
The Ph is 8.3 and ammonia level is 0, the salinity is 1.024

 

The green hair algae is probably not enough for him. There are better choices of sea slugs that effectively eat GHA, like the lettuce sea slug.

With all that said, sea hares and sea slugs do not have long life spans in the average home system.

Your system is young, and GHA is commom in newer systems. You can beat it if you use only ro/di water, keep feedings light, do routine water changes monthy or bi-monthy, run some GFO. Basically just reduce introduction of phosphates and excess nutrients to the tank.