It's a sign of error handling, in this case of the type “continue without rollback”, meaning that if the rest call fails it will follow the path downward without undoing everything that has been done until that point. Check out the documentation.
This icon appears when right-clicking on the REST activity, selecting ‘error handling’ and choosing ‘continue without rollback’. You can also reset it there. However, it’s best practice to always have an error flow out of any REST activity.