Stem Rot and Algae

[Dr4twenty]

Hi, I'm a first timer and am having some plant problems.

I went to check on my 4 week old seedlings yestrerday and noticed the base of some of the stems was narrow and brown. 3-4 of my 12 plants look like this. Aside from the stem, the plants look good and have been growing like crazy. Even my biggest and most beautiful plant has this. I picked one of the affected plants up to get a closer look and when I did, the plant fell over and the stem broke. Some white stuff that looked like mold oozed out. I removed the plant and threw it away.
I also noticed a little algae growing on the rockwool of some plants. Mainly on the smaller ones that don't have enough foiliage to cover the rockwool. I cut small black plastic squares and used them to cover the rockwool and block light. Not too concerned about the algae but the brown stems are freaking me out!

I'm wondering if I should remove the rest of the affected plants in case this is some sort of infestation or can I save them?

Here's some info on my set up. Plants are in 4" rockwool cubes in 1 gallon pots with hydroton. I am running ebb and flow, using general hydro nutes and was watering 3x a day for 30 minutes each. The water is just high enough to touch the bottom of the rockwool. My pH = 5.5 and I'm using purified water. Room temp is 70-74, humidity is 50-60%. Have fans on the plants and the room is well ventilated.

From what I have read, I am overwatering which could be causing the stem to rot. I'm cutting back to 10 minutes twice a day.

My air stone has been set to only come on when the pump does. I'm going to switch it to be on constantly to help increased dissolved O2 which can help with algae and root rot.

I'm also going to give the affected plants more support. They have little bamboo skewers holding them up now and I'm going to double or triple them up so the plants can't fall over and break.

Finally, I'm taking a trip to the hydro store later to get some meds. I heard hydrogen peroxide and/or rebound might be good.

Any suggestions? Can I save these plants or should I get them out of there before this spreads?