A mouse-of-the-seas is seen in the Shopping Trip board, in which he is described as "A bright-white-skinned humanoid, bald-headed with a russet mane growing from his shoulders". This particular one is dressed in "a grass skirt and wellingtons". 
Mice-of-the-seas appear to have insect-like mandibles in addition to, or instead of, humanoid mouthparts.
The reason for the species name has not been mentioned. It is speculated that they have an association with water or fish.