Muumuus are small blob-like creatures with two eyes that come in a variety of colors, such as red, blue and yellow. It appears that they can vary in size, with some being small enough to fit in one's hand and one in particular being the size of a human child. They also have a rather alien-like appearance, as noted by Tatsumi Oga when he first saw one.
Muumuus can float slightly above the ground, although they do not do this often. Muumuus can apparently also be used as provisional bodies, as was shown by Furcas, who uses his Muumuu for such a feat and is even able to speak through it despite that Muumuus are shown to only make "Mu" or "Ma" sounds. Also of note is that Furcus' Muumuu has two tiny arms, but whether all Muumuus have this or just his is not mentioned.