I started baking last summer, and followed a guide to make a sourdough starter which I have been using since. It always looks and smells a bit strange when I take it out of the fridge, but the last time it seemed more extreme. There was about 2 mm of gray-green hard stuff on top, but below it everything seemed normal. There were no "fur" on this top layer, it was just a bit moist with thick liquid that had almost dried out. I forgot to take a picture of it before i mixed it all together and added some flour and water to see if it was still "working". Eight hours later it was full of bubbles and smelled a lot better. There was a few dryer spots on top with fine white spots, but it could be residue flour that didn't get moist during the mixing. My question is if there could have been any mold involved in this (would the starter still "work"?), and if it's safe to use or if I should throw it out and start a new one. If there was mold, how long will it take before it starts growing on the top again?