How to get rid of white spots? Ummm - wash the sheets!
Seriously though, Atlantic Reefer pointed you in the right direction. Tangs are famous for contracting ich. My achilles has a case of it now. Just got it out of the tank and into QT today.
Hypo or Copper are the only things that really help. I think both are covered in that thread but hypo is by far the easiest and safest method.