Mosaic isn't easily identifiable as a Biomes per se because it's actually a small area comprised of many different Biomes. There are a couple of characteristics unique to the Mosaic though such as the fact that there can actually be blank holes (essentially single tiles that don't exist and through which you can see water). On top of that, this is a good place to find things like Gold, Stone and Flint just sitting on the floor waiting to be picked up.
Other than the few resources that appear and don't respawn, there's not much of value here. In fact, it is usually devoid of other normal Biome features (such as rabbit holes) and is interspersed with Rockyland that prevents planting so it isn't particularly useful for making a settlement either.