I would release the choking string at the base of the stem just right now immediately asap. I would spray that place with peroxide and treat that plant with some fungicide (U R in America so you are just fine) and I would keep the medium most all the time since this is not soil. you have sprayed that plant with “sulfer”, can you tell me why and what kind of sulphur it was? If you are sure there are no pests and no mildew, you do not need citric acid at all. I would wrap the bed in some mesh to prevent gnats to go in. Thats what I would do