Corroded Shower Floor Build Up
Such build ups virtually may exist around any shower drain strainer, grout, fixture or on shower or steam-room tiles. Owners are often confused about how to get rid of these issues on marble. In the most heavy cases the professional attention will be required.
It likely can be either hard water deposits (like on bottom of teapot) or kind of adhesive (or part of its chemical composition) what was soaking up through the grout pores or micro cracks in grout. More likely it is happened little by little “growing” during a few years.
Build up was removed by sanding out then marble polishing to original finish and re grouting affected lines. It would not be possible to properly fix by not professional, the marble under the buildup was deeply corroded and deep sanding was required. The grout was damaged too.