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 connect the afterthought fragment to the main clause in front, like this:
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.
Unfortunately, you cannot connect these two parts with a colon. Review the rules.