I am far from an expert but have you had a Marimo moss ball in that tank at any time? it looks like the Cladophora algae that those are made of.
Roll it all up and make moss balls with it
With that in mind, I have no idea how to stop it from growing in your tank.
I bookmarked this earlier because it looked to have a lot of useful information about algae.
Last edited by Darkblade48; 06-20-2013 at 10:15 AM.
Reason: Please use the edit function for back to back posts to keep threads cleaner