Green green algae indicate excess moisture and compaction of the medium. And are they actually sitting in water? They are also overcrowded for their age and size and thus are suffering nutritional issues. They are long overdue moving out of that little container.

You need to split the seedlings up and transplant into individual pots or into the ground asap. After suitable hardening off of course.

I'm growing herbs to sell the seedlings at the farmers market. I did plant some of them into larger pots and they seem to be doing better. Interesting comment about the algae. This is the first year I've tried growing seedlings in soil blocks rather than plastic cells. Because the soil doesn't have anything to contain it, it is more compacted. I took this photo as I was watering them. I water from below, allow the block to soak up moisture and then remove the excess. I don't let them sit in the water. I didn't realize how much oregano likes drier conditions though so I will back off on the watering too.