there are different types of algae in salt water blue-green,brown,red,hair, bubbles,coralline(sp?)some you want others you don't,but the most common are green and brown... to answer your question,algae is a sign on how your aquarium is doing.green is a good sign that you have a healtly tank and yes it good if you have fish that will eat it for example:
algae blenny,tangs,different angels,snail..
brown mean that its time for water changes,you either have dead spots(circulation),overfeeding,filteration issues (not enough overchange).and no fish will touch this stuff.