what is this?

To me it doesn’t look like bugs or classic PM, more like stress on that one plant – a mix of slight nutrient / pH issue and maybe sensitivity to light. The grey interveinal areas + faded, dry-looking edges on the top leaves only usually point to: • mild overfeeding or pH drift, causing a bit of Ca/Mg/K lockout in that plant, • possibly combined with light stress if that top is closest to the lamp. 17°C night temp is a bit low but not catastrophic by itself, especially if the other two are fine. I’d: • check your input and runoff pH (aim around 6.2–6.5 in soil / 5.8–6.2 in coco), • back off the feed a little for this plant or give one watering with plain pH’d water, • slightly raise the light or lower intensity if this top is getting more than the others, • watch the new growth – if it stays healthy and green, it was just a one-off stress episode. If it keeps spreading to new leaves, then I’d suspect a continuing pH/EC issue rather than the 17°C night alone.