Monanchora arbuscula is an encrusting or bushy demosponge which is dark red or bright scarlet on its inside and its surface.

It forms either a low or tall mass, and individual organisms can have a bush, round, or fan shape.

Its has scattered openings (oscula) which are surrounded by a white collar in a star-like pattern.

There are a few oscula which are on the ends of short tube-shaped lobes, and there are many knobs (lamellae) along the surface.

It may encrust different species in shallow reefs, such as dead corals, mollusc shells, and gorgonian axes.