Placing a large amount of smooth sandstone slabs in an area will cause the map to register it as a strange teal color. It is possible that this is intended, but I can't imagine why it would be. I expected smooth sandstone slabs to appear the same color on maps as other regular sandstone types, which is a sand-yellow color. In the image, you can see that the first player is positioned facing a small build primarily composed of sandstone/sandstone slabs. The second player is standing in front of a map that better illustrates the first player's position.
How to replicate:
Place a large amount of smooth sandstone slabs in a flat area, then open a map to see its color.
Edit: A similar issue occurs with smooth stone slabs, which appear the same color as mycelium on a map.