Your pH is really low. My money is on that. Go out and buy Seachem's Marine buffer - it works great. Gresham and I were talking about this on the phone not long ago. In his experience he thinks the nasty algae doesn't do as well with pH in the 8+ range. Regardless, you should correct the pH. EDIT - This pH could be caused by overfeeding and not doing water changes. Salt mixes play a part here as well.