You might be correct when it comes to English, but the Swedish sentence could not mean that.
If you'd want to express that someone dresses up as a moose in Swedish I would suggest;
A) to say "Han klär ut sig till (en) älg" - "He dresses up as a moose" or
B) "Han går som älg" - omitting the "en" in this sentence gives a different touch to it and, knowing the context, I would understand this.
Anrui already answered this in an earlier comment on this page: https://www.duolingo.com/comment/5540608$comment_id=8454806