Yes, celebrities are afraid to support Bernie Sanders publicly. Socialist is still a bad word and an insult. Celebrities are afraid to support Sanders because they don't want to be labelled socialists or left wing nuts. It is the safe choice to support Hillary as she is the more moderate candidate.
Bernie Sanders has a wide array of celebrity supporters that have been completely unafraid to do so publicly. People have to realize that not all celebrities are interested in publicly endorsing ANY candidate. This has nothing to do with the candidate, but more to do with the sensitivity of politics. Sometimes it's best to just keep your opinion to yourself, especially when you live in the spotlight.
No one is afraid to support Sanders if they want to support a candidate publicly. This is because Sanders has a foothold on liberal voters in the country and Hollywood is extremely liberal. If anything, that sounds like it would make the celebrity more popular in the eyes of fans and the industry.
Sanders has a lot of traction among young ABC Americans which is one of the most attractive demographics for commercial operators, including celebrities. An endorsement for Sanders from a celebrity would be likely to raise their standing in the eyes of his supporters and wouldn't be likely to lose them support from people who do not support him. There is nothing to suggest that a celebrity would be scared of supporting Sanders in public.