Aquariums: What is this?

I’m no expert but it looks like biofilm to me? It’s bacteria and algae feeding on nutrients in the water and creating this layer of goo as it grows. Sign of a healthy tank, though it also means that you’ve got plenty of nutrients on there and may want to cut down on the feeding a bit.

It’ll go away on its own, especially if you reduce overfeeding the tank, though another option is to introduce species that eat it like plecos, shrimp, or snails.