Hello and thank you for your inquiry.
Some people like to have UV sterilizers on their tanks, others don't care of them. There certainly are pro's and con's to them, but it really comes down to whether your feel the pro's are better than the con's if you want to add one. I have linked an article below that will hopefully help some:
Blue tangs are good at hiding, especially when first introduced into a new tank so I don't think it is acting out of the normal. Just make sure you offer plenty of greens in its diet and hopefully it will start coming out more for you.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service