I thought this might be under-watering but have just noticed these small white flecks (first photo) - they can be brushed off but are covering the entire plant, even the healthy leaves and stems. They seem too small to be scale insects and I can’t see any spider mites. There are dark brown patches on a lot of the leaves, and one of the stems has started turning brown too (second photo). There are also small (approx 0.5 mm) brown/white dots that are discolourations of the leaf itself (third photo). I've had this monstera for around three years and it's been completely fine until the last few weeks.
My flatmates monstera has had a similar issue. It has deteriorated over a few months and is now completely covered in large brown patches. I'm not sure if it also had the white flecks/small dots. Wondering if could be a bug that has spread.
Very grateful for any help!