He is Acting Chief Officer, is a title, a job/role title and all words have capital letters. John Brown is Acting Chief Officer on the ship HMS Italki. The other is describing what a person is doing, Mary Jane is acting as Chief Executive Officer today as John Brown is not here. Acting in this sense is working/doing something that normally is done by someone else.

"Acting Chief Executive Officer" would be a job title, like "interim CEO". "Acting as Chief Executive Officer" would be taking on the roles and responsibilities of the CEO, without necessarily getting the job title.