I think I have Algae!! Newbie needs help please. Thanks all

I have algae just starting on my babies. They are seed started and are popping out their 5th node today. I started them in clear party cups, just like the red ones only clear so I know that the light caused the algae to start (I did some reading before I posted but I'm still not sure). Yesterday I bought a bunch of red party cups that I just put the clear ones in so that there is no light getting to the roots but I can remove the red cup to observe the root growth (kinda cool for future starts). I was just bragging them up at my local grow shop about how they were pure white and that they are rooting very well. (open mouth,insert foot). My question is will the algae just die off now or will it spread? Do I need to do anything to stop the algae growth besides just blocking the roots from the light? This is my 1st indoor grow. Any help would be greatly appreciated.. Thanks all

My set up and nutes:

Set up: 4' x 4' x 5 1/2' high, home built with 1/2" plywood. I caulked at every spot where any boards met and painted the interior with a roof coating made to reflect light (90%), it also has an anti microbial/ anti mold/mildew. I also covered the entire interior with Mylar. I have a 4" inline centrifuge blower fan pulling the exausted air to a carbon filter (the exaust is at the top rear of the box). I have an inlet hole for fresh air at the opposite corner at the bottom of the box. there is a 4" aluminum hose running to just under my window for fresh air. I should mention that I live in Alaska and get cold air coming in. By closing and opening the window a small amount I am able to regulate the temp at around 73*F. My light is a 400 watt MH with a Hydrofarm hood. The exaust is vented from just above the hood of the light. I ran some 4" aluminum exaust from the blower inline fan to just above the hood so that the fresh air would run up through the plants and through/around the light. There are no light leaks at all except a small amount of ambient light that comes in from the inlet but very, very little if any.

Nutrients: Pro mix soil, Botanicare PRO grow 3-2-4, Botanicare CAL-MAG, Superthrive, Ph up. I just bought but havent used: Dyna-Gro K-L-N, Olivias cloning gel, Europonic Rockwool conditioning solution, some rockwool cubes, a flat to hold the cubes and a Hydrofarm starter heating pad to keep them warm.

I have been Ph ing the food to 6.5. I have been useing a liquid Ph tester so I'm getting it as close to 6.5 as possible.

I would really appreciate any help and advise. Thanks everyone. I have some other questions about cloning but don't want to overload with my first few questions. Thanks again for your time and help.

I once had an algae problem -- if it's at the beginning of the problem you should be fine once you block the light. The lack of light will kill the algae, plain and simple. I think if it was a problem left untreated it could cause bigger issues, but at this stage just block the light and forget about it.

Mine looked like this. Nice and slimy:

Yea it just started the greening yesterday and I convered the clear cups with red cups so no more light to the root system. I have another question if u dont mind. I have 18 seed starts and they are on their 5th node. With the limited space I have is it possible to transplant them into 2 gallon pots and leave them in the 2 gallons until they are finished? I dont have them sexed yet and I also wanted to know if I should top them before transplant or transplant and then top them? I have 2 larger females that are about half way through veg so I'm trying to figure out the best way to do this grow. Should I clone off the 2 larger females (very good but unknown strain) or go with the lemon haze seedlings and take my chances on female or male. the other option is to just keep the 2 larger fems in veg longer and switch them all to flower when the seedlings get going more..?? I have no camera for pics but I will take some with the computer webcam when the light goes off tonight. Thank you much for the help and advise..