may want to put this on down in the gardening section we're a bunch of cannabis growers here you may get more responses from the gardening section
Hey! Cannabis lover and succulent lover here. Looks like rot to me. the one struggling is an echeveria, they are much more sensitive to overwatering than the other succulents you have in that pot. The white-spotted one is a haworthia, the other is a jade. Anyways, unfortunately the echeveria is a goner. I would remove the dead plant, and consider repotting the other succulents as there are bad fungi living in the soil due to the rot. I think the other two plants can live in those conditions, but succulents generally struggle when placed in a ceramic pot, even though the inner plastic one has drainage. The ceramic still prevents water from fully draining. If you were to get another echeveria, I would suggest a terra cotta pot. this will allow much more airflow and drainage. I know this link is cannabis growing focused, but it has some great info on good soil types and pot types for various plants and explains how soil and water interact: Everything You Need To Know About Growing Cannabis Indoors Best of luck! Feel free to comment any other questions back to me