RubberMaid Containers, and the likes, leech out phosphates from the plastic after a short period of time. If you are mixing and storing your salts in such containers you may want to consider using High Density Polyethylene. The material is FDA and USDA complaint and should help reduce phosphates from this unexpected source. The best deal I found is Baytec Containers: They have reconditioned 55 gallon drums with opened tops and covers for $79 (smaller sizes are available too) and no shipping which will save you a bunch. Check with one of our sponsors, as they may have something smaller at a good price too.
You can always try using the toothbrush on your live rock as this type of algae seems common on new set ups which are early in their cycles and will go away with good feeding habits and 5% water changes twice a week. My experience with non-live sand is that it creates a lot of diatom blooms for the first month or two and then levels off. It' best to let the diatoms cycle through and not freak out and clean the sand a lot unless you start getting cyno. I understand that diatoms compete with cyano, so the diatoms should keep the cyano in check (just giving you a heads up) Here's the link to Baytec http://www.bayteccontainers.com/55galoppldr.html
Good luck.
Update: 9/10/09
Strike the Baytech comments. I canceled my order with them. Not only are they not the best deal, but their sales practices leave little to be desired. Instead, I found brand spanking new HDPE 55 gallon barrels for $50 plus tax 8.5% & shipping (apx $48) Here is a link to their site-Kristie has been great. http://www.qualservcontainer.com/catalog/i25.html. They are FDA and UN 1H2/Y130s Complaint too.
Update 09/16/09
55 Gallon HDPE Bartrel arrived today, as promised; and it is totally awesome. http://www.qualservcontainer.com/catalog/i25.html and Kristie have been great, and they have exceeded my expectations.