It's a newer version of the 'velious' or 'transparent' UI added back sometime in the humble beginnings of EQ.
It went through different stages. What we have right now on Green Beta is a much later version. A few people, with the guidance of Rogean/Nilbog, are working to 'classic-ize' the velious UI and will be distributed when they deem it's ready. (this is my interpretation of what will happen)
See the image below for an example I took of the trilogy era UI, taken on the eqclassic server a few days ago. This is the version they're sort of aiming for.
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