I bought my TRX last summer, and the cable seems worn, right where the plastic piece that goes over the cable slide rod rubs the cable through the drawing and shooting cycle. I think it looks more like only the coating over the cable, but should I replace it to be safe? Also, should I replace the cable AND the string or only the cable?

I would replace it to be safe myself.If you are doing alot of shooting with your bow,then chances are good your factory strings and cables have stretched anyways.Its almost a good practice to replace every year or couple anyways.
Another thing to consider also is to replace the cable slide to a sims or hicks or something in that calibre,teflon is a little easier on the cable and a smoother draw as well.
Have the string checked at the shop as well,maybe it only needs a new cable,a a few twists in the string-------Good luck

Yea, its a good practice. I usually replace my string every year and my cables every other year (if they will hold out that long)

For one if your string and cable's have never been changed ,I'd go ahead and change both just to get a fresh start .I wouldn't change one cable without changing both at the same time .If your string show's wear, it definetly won't hurt to change it on it's own as long as the cable's are fine ,but being that you have one bad cable ,I'd change both at the same time .Also as Extreme1 say's you can get a teflon slide that will improve the wear on your string and cable's and also eliminate's some noise issue's as well.

nubo