The green on the soil is algae. The algae won't hurt anything but it is a sign of soil being too moist for too long. I don't grow dragon fruit plants except as a rootstock for a grafted cactus and I don't have a London climate so my suggestions might be a bit off. However, I would improve the drainage of the soil mix. Dragon fruit cacti seem more tolerant of moisture but I've seen them rot from the soil up because of over watering.

Is this London, UK? If so I’m sure you’ll know this will not survive the winter on the balcony. It’ll need to come indoors.

If you are in central London you might have a microclimate which will allow it to just about survive but it won’t cope with frost. Even the city centre gets these occasionally. Have you recently moved to London? Have you experienced winter there yet? A plastic greenhouse would probably help it struggle through. But it will be pretty scruffy by next spring. Much better to bring it in during December, January and February.

Cool temps and overcast really aren't good. These don't do anything once the temp drops below low 50's F. It's easy to rot in temps lower then that.

Well, it’ll be below 50 in London most days in winter. And frequently overcast.