proflex is a great caulk. I have had bad luck with clear...it just is worthless. Use Black or White. I found out the clear is the only one allowed for use in California...that should tell you something...this is on Geocells website.
When searching the SDS and PDS of the various Geocel Proflex sealants, I get the same .pdf caution sheets for all colors of their Proflex products. Maybe they have changed the formulation recently.
Typically, the CA use cautions relate to that list of chemicals that CA has found to induce cancer. That may be the benzene ethylate or other solvents used in the formulation.
FWIW, I've used Geocel's 2300 series of sealants, almost exclusively the clear formula, and the stuff sticks like no tomorrow. Geocel also makes other polymer sealants for truck and trailer construction and I'd bet those are good, too.