From the Super Mario Wiki, the Mario encyclopedia
From what was seen of Quirk in "Stars in Their Eyes", it is a very desolate world, with no plant life seen on it, only red deserts. Several rivers are also found on Quirk, the water of which, is orange. The gravity of Quirk is also much lighter than the gravity of Earth as, without something to help weigh them down, Mario, Luigi, Toad and Princess Toadstool were shown to float upwards upon stepping on the surface of Quirk.
Only one native species of creature is shown to inhabit Quirk, the Quirks, small, green humanoids with trumpet-like noses jutting from their faces.