Droppings of some sort- my suspicion being from a rat or mouse.
My son found this on the beach near our boat dock today, we have spent a huge amount of time on this beach over the last 20 years and have never seen anything like it. I vaguely remember seeing a photo of one of these in a marine biology textbook and at first wanted to say that it was a sea biscuit or echinoderm of some sort but it is really unique! I didn’t see it in person but he also has some good video of how it was moving. Amazing! I’m so interested in knowing what they eat and who eats them. Any insight would be appreciated!