I have tried keeping Anubias nana as well in my tank, but I never managed to keep it healthy. As it was the slowest grower of all plants, it got covered with BBA before any new leafs were grown. I removed mine, but hope you'll have better luck solving your problem.
You don't mention how long you've had the Anubia in the tank. They take some patience as they are slow to grow and slow to acclimate themselve and get established, and in the mean time can pick up algae. I'd give them some time. I plant mine in a shadier area of he tank and that seems to help with the algae as well.
I guess kvr is right I have the nana in a 50 gal. with a medium fish load of tetras. The nana has been in there for about 10 months, with java ferns, cryptones, and a lot of java moss. No Co2 or ferts.