I have helped lots of people with Green Water (GW) and have a sort of a canned reply. Read the below text and you'll see why you have GW, why changing water won't make it go away, and lastly how to get rid of it. It's important to note as Kyle already pointed out, GW will not harm your fish.
The situation that causes GW (Green Water) is usually a combination of high NO3, PO4, and mixed in some NH3/NH4+. Substrate disturbance is usually the culprit. What happens is the algae (GW form) will flourish off of the NH3/NH4+ and PO4. Remembering that algae can consume PO4 easier than plants because of their thin cell walls, the algae uses up the NH3/NH4+ and PO4, but it doesn't go away...because algae can quickly switch which nutrient it scavenges...it moves to NO3. So you can see why water changes will not rid a tank of GW. Nutrients can be reduced very low in GW and fairly quickly by the GW algaes, but they can scavenge other nutrients...iron and trace elements. So, it's very common for the GW to solve the situation that causes it to begin with, but that won't eliminate the GW, for the reasons I've allude to. Four methods exist to eliminate GW. Blackout, Diatom Filtering, UV Sterilization, and Chemical algaecides/flocculents. The first three because no harm to fish, the third one does.
The blackout means covering the tank for 4 days, no light whatsoever is allowed into the tank during this time. Be prepared, killing the algae will result in dead decaying algae that will decompose and pollute the water. Water changes are needed at the end of the blackout time and NH3 should be monitored also.
Diatom filters can usually be rented from your LFS. This is my preferred method. Personally, I use my Magnum 350 w/Micron Cartridge coated with diatom powder. Diatom filtering removes the algae and doesn’t allow it to decay in the tank. You do have to check the filter often, if you have a really bad case of GW the filter and clog pretty quick. Just clean it and start it up again. Crystal clear water usually takes from a few minutes to a couple of hours.
UV Sterlizers will kill free floating algaes. They are expensive and don't remove the
decaying material from the tank, if you can afford to keep one they are handy to have around, though not as useful IMO as a diatom filter.
I hate the last way, the flocculents stick to the gills of fish, while not killing them it does compromise their gill function for quite a while leaving them open for other maladies.