Today I tried to sign up at HealthCare Marketplace and was informed that I may be eligible for medicaid. On Feb 15, the marketplace shuts down and I won’t be eligible to purchase insurance coverage. Can I cancel my insurance I purchased through the marketplace if I should be determined eligible for medicaid?
If you filed an application and have been deemed eligible for Medicaid, but are awaiting an official decision, then you don't have to do anything. If Medicaid is denied that should trigger a special enrollment period that will allow you to enroll in a Marketplace plan. Being denied for Medicaid also triggers an exemption (if your state didn't expand). If your state did then you'll have a coverage option no matter what.
The above being said, you'll need to be proactive about your application. Double check your account is setup correctly and your application status is pending.
Learn about Medicaid Expansion.
Learn more about Special Enrollment Periods.
Learn more about Exemptions.