Phlegm is mucus produced by the respiratory system, excluding that produced by the nasal passages. It often refers to respiratory mucus expelled by coughing, otherwise known as sputum. Phlegm, and mucus as a whole, is in essence a water-based gel consisting of glycoproteins, immunoglobulins, lipids and other substances.
No, it is not makruh to swallow phlegm.
And Allah knows best.