More than likely, when the plant was being watered, the sheath was trapping the water.
Now that it is opened, it should dry out and take care to keep the area dry when watering.
You could open the sheath more but trying to cut it off around the stalk is tricky and I don't believe necessary.
Why risk damaging your beautiful flowers by working around the stalk with sharp things
Wait until your blooms are done in a few weeks and cut the sheath off with the spent blooms/stalk.
You might spray a bit of physan then if you are worried.
If I have a sheath that begins to turn brown without any flowers in bloom I do remove it like others mentioned.
Overall the plant looks great! Gratz!