We were cleaning a house the other day that had been neglected for quite some time. I've seen others post pictures in the past of red algae and mistaken it for rust. This is an area at the base of the house where this very hardy, stubbron form of algae was present. It can take several applications of full strength chlorine, and long dwell times to completely remove this form of algae. It will usually turn a brighter orange color during the removal process, and eventually disappear completely. this is actually pretty common here in Florida, but in other parts of the country this kind of algae is probably not seen very often.