Seaweed has washed up as one of summer’s trendiest ingredients in beauty, skin and hair-care products. Touted for ages for its healing and cleansing properties and used widely in salon and spa treatments, seaweed has made its way into shampoos, face washes, makeup and anti-cellulite products that can be bought at drugstores and mass-retail outlets. “You will find all three major groups of seaweed …