According to radiology websites, hypoechoic cysts are usually endometriomas or teratomas (dermoids) both
of which are benign (non-cancerous)
cysts.
Of course, imaging is not exact so they cannot know for sure until they do surgery.
The CA125 test is not a good tool for diagnosing cancer. Benign conditions, including benign cysts, can cause an elevated result.
The good thing is that your gyn plans to do a cystectomy (cyst removal) which would preserve your ovary.