I presume you're talking about the Gold Line light rail in LA, if it's a California line at all. Gold Line in Sacramento doesn't use flashing indications for signals that I've seen (riding it for over 10 years).
Sounds like they might be using some flavor of the old Santa Fe indications shown in the PDF from Trains (link below). Perhaps appropriate, given who used to run where LAMTA's Gold Line is now. Flashing green is used by some railroads to indicate "limited" speed - less than maximum but not "medium" or slower. More informed googling might improve the answer.
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trn.trains.com]
As somebody mentioned here before, there is no standardization for signaling among transit lines - you might need the local rulebook to know for sure what it means.