Karin, I'm actually pretty sure that it's an attractively-placed stripe of cyano, believe it or not. From what I've read, it's not uncommon for seahorses to grow a little algae on them because they move so slowly. Kinda like tree sloths, only in saltwater...

The ability of an individual seahorse depends on what kind of chromatophores it has. I don't THINK these two can do red, but we'll see. I can't wait to find out what kind of color changes these two go through!