You have a lot of algae, and it does look like it is on the roots along with the debris. Because of this, it is reasonable that the algae and Duckweed are competing for nutrients. The browning could also be sunburn. It may be just an illusion, but the algae on either end of the aquarium looks healthier.
Surface tension is holding the bubbles in the duckweed because that water is not moving on the surface. It could also be from a contaminant, your water conditioner, ammonia, etc.
I don't think your water is as stagnant as you think it is. Some of the sideways Duckweed could be trapped in the surface film or actually moved by the water. Your side mounted filter is moving water down and across the tank until it circles around. I don't know if the DW inside the ring is left over from when you installed the ring, or it got blown over or under the ring. Either way it looks good, and I'm a big fan of the side mounted filters.
Take all of this with a grain of salt, because it is coming from someone who struggles to grow Duckweed. My best efforts so far are inside a protective ring, behind a floating mass of Hornwort.🦆