
Continuity mistake: After Carina tells Jack and the others that she's an astronomer which has nothing to do with donkeys, when Jack threatens to kill Henry in the shot facing him he's holding his left hand up pointing at Henry, but in the next shot facing Henry it's Jack's right hand up pointing at Henry.
Answer: He's probably worried that either the crew will attack or at very least frighten Henry. Or he just doesn't want Henry to see how horribly the curse can affect people.
But because Will is ferrying souls to the after life, like Davy Jones should have done for the full time, he was captain of the Dutchman. There is no curse that would make the men look like sea monsters. They only started to look like that because Jones denied his duties after 10 years of service, when Calipso didn't show up. So the crew would look like normal men, so Will being worried that Henry will be frightened by them, can't be the reason he started to panic.