Hi Anna, mold or a pink or orange streak or tinge are generally signs that your starter isn't healthy enought or mature enough to resist such invaders. However, a small spot of mold might be something your starter can recover from. I would recommend feeding some of the healthier-looking starter from the bottom (mold grows from the top down) twice a day at room temperature for a few days. If your starter begins doubling nicely within 6-8 hours after a feeding, has a pleasant aroma, and show no more signs of mold growth, then you should be fine to start baking with it again. Be sure in future to give your starter a few hours at room temperature after a feeding and before refrigeration. It's important for the starter to start bubbling and fermenting a bit before you stick it in the refrigerator. 

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