From my personnal experience, its a water+light problem.
When i look at your picture, your heli do not get enough light. The plant is very green.
Light = energy, so when you dont throw enough light, the plant cannot deal with minor infections and this can take over your heliamphora.
So for me, your issue is clearly a lack of light+lack of ventilation.
Of course, like i said, i speak from my own experience.
Feel free to disprove!