I only met him twice, so I don't have much to say, but let me share Alan's message instead, since he knew him well: https://discourse.julialang.org/t/cleve-moler-rip/137235.

How very sad. Whilst not my piece of software of choice, it is definitely still used by plenty to do cool work. I do think it will eventually be surpassed by Python and Julia but it still has it's place right now.