Revealing mistake: When Jack stumbles out of the vault he questions the crowd as to why he's there, and as he holds up his left hand we see the tattoos on his knuckles, but when Jack says that he needs just a moment to clear his head, those knuckle tattoos on his left hand vanish and a ring appears out of nowhere. Then the tattoos reappear and the ring vanishes.
Revealing mistake: When the bank vault is hitched up to the team of horses note the position of Marty's white horse, and the buildings around the bank, because two of the following wideshots are flipped. First, after the ropes get taught when it starts pulling the vault, the shot of the team is flipped; and second, after Jack gets on his knees just as the bank moves around him, once again the shot of the team is flipped.
Revealing mistake: During the bank chase at the beginning the bottom plank is a material skirt which moves forward and backwards revealing in some shots the wheels underneath the bank.
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.