Cool! I love People of the Black Coast - just a very grim and dark tale with powerful imagery typical of Howard. The scene where the guy discovers the girl's hand lying on the beach stays with you for awhile.
I've wondered about the many similarities in the stories too (I was going to use the crab monsters in my photo manipulation but couldn't find a good picture), the movie was 1957 and howardworks has 1969 as the first publishing date of PotBC, so I guess it's coincidence. The only other explanation I can imagine is that telepathic giant crabs are real and beaming their bizarre emanations into the unconscious minds of certain gifted individuals.