Mine eats any food that touches the water, but has never touched my corals or GSP.
His dorsal goes up if anyone walks close to the tank or makes sudden movements, unless he recognizes my glasses, then he is happy and colors back up.
If I have contacts in, I can talk him into calming down.
If the hair algae is long, mine won't eat it, but if you remove the long pieces, he will finish the job. My blenny does pick on him occasionally, leaving round suction marks on his sides. I hadn't figured what that is about... maybe just a bossy territorial Blenny!