Hi Alyssa, since your starter is a mature starter that was just neglected for a few days at room temperature, I think it's very unlikely that it would develop any sort of bad bacteria. Mature starters are typically very resistant to such outside invaders. If your starter is rising well, has a pleasant aroma, and shows no signs of pink or orange coloring, I think it is totally safe to use.  Most likely the skin on the top was a result of a little too much airflow causing the surface to dry out, and this might look a little darker because of oxidation. 

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