How is impostor syndrome different from having low self esteem?

Er, one is a fear of being discovered as incompetent, and the other is low self-esteem? ;-)

Obviously, having a low opinion of your abilities would be a prerequisite for fearing that others will come to share that opinion. So, I'm not sure I understand your question.

"Low self-esteem" is usually interpreted to mean, "having a lower opinion of self than is merited" -- so if you fear discovery of your incompetence, the incompetence itself could be either actual or imagined, and only in the imaginary case would the "low self-esteem" label also apply. ;-)