How to remove this pipe plug?
It looks like a plug in a barbed fitting for hose with clamp attachment. When not being used, the drain is sealed by the plug.

I would apply heat to the barbed section causing it to expand slightly and loosen it's grip on the plug. A hair dryer might work, if not an electric heat gun set to LOW worked all the way around the barbed section for a few minutes then try turning it out again - and remember "righty-tighty, lefty-loosey".