Moss

Moss is an object that appears and spread throughout rainy weather. Moss stops you from placing anything on that block, and won't be removed if an object is already planned on that square, and is then built. Moss won't place on prebuild objects, only blank rafts. The only way to remove it is by shaving it. The moss will then drop Moss Vines, which can be crafted into Cloth.

Moss spreads very quickly and requires a lot of micromanagement to remove. If you build a roof over your blank rafts then moss won't spread onto it. Moss also only appears on the ground floor, although that may be a glitch.