The thin plastic pot you have it in along with the sphagnum moss is likely your problem. Those pots don't drain well and the moss holds moisture. I personally can't stand them. I would suspect the plant's roots are rotting. I would remove the plant from the moss to take a look at the roots, remove any rotted roots, and repot in a potting medium and pot that provides more drainage and air circulation around the roots. Feel free to post photos of what you find and we can better assist you.