Whatever it is, it grows extremely fast. 3 hours ago, there was no trace of this at all. Now..., well the pics show what I found. It's only growing in this one pot, and the media isn't bad, only about 3 months old ( bark mulch/perlite). I'm hoping it isn't harmful. I'm assuming it's just a fungi that feeds on bark, but what's weird is how it popped up so suddenly! Also, I do a monthly trichoderma treatment. The pot is 6" across for scale.