not to worry...your SH has a case of cyano growing on it. SH, being rather sedentary can be prone to various types of algae growing on them. it won't hurt them, and in the wild, it helps them to blend into their environment.
if you drop an airline into your tank (just a bare piece of rigid is fine), the SH may hitch to it and put themselves in the bubble stream to clean off. otherwise, you can take a baby toothbrush and GENTLY remove the algae from the SH.
HTH