I am not an expert on "oddball inverts" but I was looking at pictures and it looks like they have small paddles under the top exoskeleton that they use to swim around. This could be moving the sand around and causing little trails. I have some questions about how to keep them, they look cool! What do they eat (probably bloodworms/ brineshrimp?), How big do they get (2 inches?), are yours breeding, and water parameters.
Usually when you buy them they come in a kit with included food or, you can feed parboiled carrots and cauliowers and sometimes bits of shrimp, and bloodworms/brine shrimp will work too. The maximum size i would say it depends but you need good conditions or growth may take longer such as in cold water. For water, it is reccomended to use spring water that says bottled at the source other than distilled or tap water