discoloration of seedlig stem MAY indicate "damping off) a fungal problem IMPROVE air circulation around the remaining plants BUT not in a draft.....

Last night I opened up all the vents in the green house and the plants are recovering well! But I did discover some fungi growing... I realize this may not be bad fungus but nevertheless, I have a feeling I left the vents closed for too long.

Your temperature seems very high for seedlings which have already germinated. Is that a chard seedling? if so it's way too hot. And the medium looks rather soggy. The fungus could also point to damping off.

I'd cut back the water, increase the ventilation and lower the heat.

Here is a link that might be useful: Damping off.

I'd worry that 24 hours of light could become problematic as well.

Yep, temperature likely needs to go down and the light definitely needs to cycle.