I have seen this several times now, Sometimes on aluminum ridge vents, sometimes coming off the corner of skylights, sometimes coming off of these aluminum vents.  I say sometimes, because it is very hit or miss. I can walk through a neighborhood all done my the same builder, and some of then have these issues and some don't. My question is can they be cleaned? How would you go about it?

Here is a picture to help. It is not ALGAE!