Microfiber refers to the size of the threads so without more information I can’t tell you exactly what it’s made of and the health or environmental profile.  In general, it is a synthetic fabric and usually made from polyester but can be a blend of materials or polypropylene.  I don’t recommend sleeping on any synthetic material.

Bamboo, however, is also not a great choice.  Bamboo is a sustainable material but it requires intensive chemical processing to make it into a soft fabric.  I recommend organic cotton, linen or silk for sheets.  GOTS certified cotton is ideal.