Algae on Anubias


I have an A. nana that has been growing quite well up until now.  On each of
the leaves (not the newer growth) a plague of some type of algae has covered
them completely.  I am not very good at identifying different types of algae
and am not sure what it is.

The algae starts off as small individual colonies circular in shape, and
eventually spreading resulting in complete coverage of the leaf.  It is dark
green in color with a "bluish" tinge.  It does not come off of the leaves (I
have used my fingernails to try to scratch it off with no success).  The
Otos will not eat this particular type.

Any suggestions on what type of algae it is and how it can be kept in check?

Godspeed to you.