Spirulina comes in powder form...and u have to use some water to make it into like a paste then add mysis shrimp or brine shrimp to it and let it sort of marinate/soak for about 15mins-30mins then add in only the shrimps that have turn green due to the spirulina marinating...filtering off the spirulina and left with the shrimps before feeding as the spirulina will foul ur water and rise nitrate.
Tangs should be offered algae to eat for it to have a healthy diet and also it will help in their immune systems against ICH...can try Julian Sprung's algae or maybe buy red bamboo algae...I have algae in my tank to feed my tang and also I use spirulina once in a while...after my tang have totally fight off the ICH problem, normally I just feed it mysis shrimp or brine shrimp once in a while with spirulina sometimes...
My 2 cents worth