What is this yellow coloration of cotton wool used as growth media for hydroponics?

Question

I have built a simple hydroponics DWC system, using a window box with aeration. Actually a system exactly like the below picture:

I am growing lettuce and cabbage, and a week ago I have transplanted my first seedlings into the system. I have grown the first seedlings in cotton wool, as you can see in the picture below.

Unfortunately some yellow stuff have shown up, and quite rapidly grew to what you can see below. It seems to only have affected this one seedling this much, but some of the other cotton wool shows slight yellow formations also.

It does not seem to affect the actual seedlings, nor does it seem to spread much more. But I am curious of what it is, and if it can be problematic? Or perhaps the problem is just that I am using cotton wool, and it becomes constantly wet when placed in the hydroponics system?

I have started using rockwool cubes, which does not seem to be affected in the same way.

Is the yellow stuff something I should be worried about, and what is it? :)

Answer 1

The original poster says:

It seems the yellow stuff is simply some discoloration, and does not cause any problems in itself. I have however stopped using cotton wool entirely, as is has some major drawbacks. In a DWC, it gets completely soaked all the time, and the roots within will not get any air. Even if the DWC is aerated. So for me I changed completely to rockwool cubes, and I have not seen the discoloration since.

Which implies that this discoloration only affects appearance, and does not detract from the performance of the cotton-wool. Cotton is not the ideal growing medium for a water culture system, as it will quickly become saturated, and decomposes quickly. Switching to rockwool was a good option.