Captive Plug Inside Shower Drain

I popped off the drain cover from my shower floor and found a large rubbery plug inside. It is installed in such a way that its diameter is bigger than the metal shaft above it. There is a string on it. I realize that the plumbers in Hawaii do things bass-ackward, but has anyone ever seen anything like this? How do you remove it to get a snake down the drain pipe? It seems that the section of pipe that secures the drain cover is set firmly in the grout, so I am wary of trying to rotate it out (or worse, introducing a leak).
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