Here is the original item:
All my friends wanted to spend Friday afternoon at the beach. Except Michael,
the worrier, who was concerned about shark bites, jellyfish stings, and
skin cancer.
You chose to replace except with excluding, like this:
All my friends wanted to spend Friday afternoon at the
beach. Excluding Michael,
the worrier,
who was concerned about shark bites, jellyfish stings, and skin cancer.
Unfortunately, both except and excluding can begin afterthought fragments. Since there is no main clause in the part about Michael, you still have a fragment on your hands. Review the rules.