exactly what Fred said!!!!
I have a cyano outbreak right now... guess what my horses are covered in!!! its kinda cute, and its just part of owning seahorses. In nature, since they are at THE BOTTOM of the foodchain, they disguise themselves with macros, etc. It just kinda, grows on them.
If its bugging them, they'll sit in a high flow area till its gone. its going to stress them out more to be scrubbed off than to just let the algae be! Theres reports of horses with all sorts of algae: coralline, calurpa, hair... all sorts.