I blended up some skinned ginger and squeezed the juice out of it via a meshed strainer with small holes (I only wanted the juice and I was collecting the pulp for cooking). I collected the juice (200 ml) in a steel vessel, where it remained for over 1 hour. Then I poured it into the lime I squeezed for a lemonade. Later on washing the dish I found a white sediment, I use sediment that instead of residue (ginger fiber) as that was thick white coating on the bottom of plate and I had to use the sponge to wash it away. Even though the ginger juice was yellow-greenish, this was "salt like" white. What is that white thing? PS: I am very sure that it was not there before and I cleaned all the vessels & skinned ginger very well. 
Ginger roots contain ~12% starch (potatoes in comparision ~18%) and the facts you described (settled down from juice after some time, white color, dense/thick texture) match the way how starch appears when it is collected for making Knödel (german dumplings made from raw potatoes). If you want to proof it you could either treat a potatoe in a similar way as the ginger or apply an iodine test to the sediment. 