I walked right over these succulents, and @rjadams55 pointed them out to me. Their colouring blended in so well with their serpentinite substrate!

UPDATE 3/14/2017: Dudleya abramsii ssp. murina (Mouse-Gray Dudleya) is included in the CNPS Inventory of Rare and Endangered Plants on list 1B.3 (rare, threatened, or endangered in CA and elsewhere). https://goo.gl/k9NDr2