In bpd treatment, phlegm turns to be thin, does it mean it is getting better?

For the moment: Bpd in preemies (or ex-preemies, as we call them when they are older) is a chronic situation. New lung growth, along with decreased inflammation in existing lung, is what it takes to get better. There will be better days and worse days, so one can say that things are worse if the child has more phlegm, and that things are better on the days there is less or thinner phlegm.

Yes likely: Airway secretions are normally very thin, clear and watery. As an infection or chronic inflammation improves the secretions usually improve as well.