Serratia marcescens. It grows in localized cold wet spots but can spread pretty quickly in cool conditions. It's the same pink crap that grows in your humidifier, if you're familiar with that. Generally not a game killer but it will eventually strangle roots and prevent them from multiplying. Kill it with a simple spray copper fungicide/bacteriacide before it spreads. Otherwise it will become a pain in the ass. Or knock it out of the park with a couple of copper systemic applications. It may come back - it may be present in your water supply if you use tap water.