Your molly looks like it has popeye. It is often caused by either physical damage to the eye or prolonged poor water quality. When you can, it really is a good idea to have a liquid testing kit handy just to be able to know what's going on with your fish when something like this happens. Water quality issues are fortunately fairly easy to fix once you know about them! If poor water quality is the problem, clean water may be the best treatment of all. I would suggest a 50% water change for the moment, personally.
Tetracycline may help at this stage, or it may not. Using it, though, may kill some of your beneficial bacteria, one of the most important things NOT to do right now if your water quality is poor.