Mantis was the other thing that came to mind. Hear any tapping sounds at night?
When you add the new fish after QT, keep the lights off for a few hours or try rearranging a few of the rocks. That resets the territory boundries and makes adding newcomers easier.
Have read a few theories that fish can actually see color and when they see a fish of similiar size, shape and color, they see it as a competition for their food source. May not be true, but sounds good.
Tangs usually won't touch hair algae. Best way to get rid of that is nutrient control (nitrates, phosphates, silicates) through regular water changes and filtration.
Hope this helps.
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