Daniel,
That arc connector is a polyline on the same layer as the plant callout and leader line with no actual connection to the plants or the label attached to it, so you have some options.

You can of course just delete it and use the connecting arc label tool again. If the change is minor, you can quickly manipulate the arced polyline instead. Grab the grip points and move them around as needed. You can hover on a grip point and CAD will give you the option to even remove that grip point (if a plant was deleted) or to add another. You can then use the group label too to edit the existing label to include any extra plants. I usually find this to be the faster option.
Here's a video that shows (among other tips and tricks) the group label tool for editing existing labels.
Power Tip: Advanced Labeling Techniques

-Amanda