How can I make a carob coating?
Melting carob is a bit different from chocolate.  The sugar crystal formation is not the same for tempering chocolate.  One suggestion is to add paramount crystals (which is a combination of palm kernel oil and soy lecithin) to help start/control the process.  I would also recommend not keeping the double boiler actually on the double boiler for the entire time.  

Heat it up until some of the carob melts.  Remove from heat and stir.  Add it back to the heat and continue removing to stir as each bit melts a little more.  It takes longer but it allows you to control the heating and crystallization process.  

End the end, you should have a smoother and more even end result.