Your discharge timing around day 15 fits ovulation, when a brief hormone dip can cause light spotting along with normal, clear cervical mucus. This is usually separate from your ovarian cyst. A 6 cm cyst more often causes pelvic pressure or pain, and you don't have either of those symptoms.
Next Steps
Track this over your next one or two cycles and note the exact day it happens relative to your period. A pelvic ultrasound can confirm whether the cyst has changed in size and rule out any cyst-related cause.
Health Tips
See a gynaecologist sooner if the bleeding gets heavier, lasts beyond a couple of days, or the cyst starts causing pain.