That is crown rot. How did this happen? Water was sill in the stem, it remained on the plant too long, and then there was a temperature drop. In the wild, Phalaenopsis actually grow sideways so that excess water is pulled out of the center of the plant by gravity. When watering, be sure to upend your plant to allow water to run out of the center of the plant. Run a fan to circulate fresh air around the plant so it dries quickly.

I suppose this is good advice if you get another Phalaenopsis.