I'm afraid that looks like rot - take the plant out of the pot and check the roots to see if they are mushy and rotten. Take more pictures of the whole plant and let it dry out if it is wet. It is likely it has crown rot (in the plant, from letting water get in the top of it) or root rot (the roots haven't had enough air and too much water and it is rotting from the bottom up). Unfortunately rot can be hard to stop in Phals. Others will give more advice.