This video from England says about itself:
Angular or Square crab in Devon
Goneplax rhomboides, filmed at Sprey Point, Teignmouth January 2018. These odd-looking crabs can sometimes be found alive washed up on the sandy beaches of Torbay, particularly after rough seas.
They usually live in burrows on muddy/sandy bottoms from the shallow sublittoral to 100m depth.
Translated from wildlife warden Joeri Lamers on Terschelling island in the Netherlands, 28 August 2019:
In recent months I have seen more and more reports of discoveries of angular crabs on the beaches. Usually they are dead animals, but also some live ones. The Wadden Sea Society even had one in their aquarium this summer. The angular crab is a newcomer to our coast, it used to be found in somewhat more southern waters that stay warmer on average, and therefore it benefits from the warmer water.
This weekend I found a dead specimen on the beach at beach pole 8. Earlier, I had already found loose carapaces, especially after the hard January storm in 2017. Actually since that time you see the discoveries increase, including through waarneming.nl. This summer, I think it suddenly became a lot, and also spread over a large part of the Terschelling beach. Most finds are between beach pole 3 and pole 7, but they are also found at pole 16. I like to hear new notifications, put them in the comments under this blog post.
This crab lives on average in reasonably warm waters, especially south of the English Channel and in the Mediterranean Sea. The North Sea was always too cold for them because this is a shallow sea that cools quickly in the winters. Deeper water maintains a more constant temperature. The water in the North Sea gets warmer on average, with opportunities for these and other species.
In 2016, this species was found for the first time on a Dutch beach; on Ameland island, east of Terschelling.