What is responsible for the underwater waterfall illusion of Mauritius Island?
The cause for this phenomenon is sand and silt moving down an underwater cliff as you guessed.  The geography that makes this possible is as follows.  The island of Mauritius is located on a giant plateau that was created by seafloor spreading a few million years ago.  Most of which is under 8-150 meters of water.  Where the waterfall effect occurs there is a huge drop-off which drops down thousands of meters.  Because of the islands angle when the waves beat upon the shore and displace sand, the drawing out of the waves creates, you could say, a down pulling  effect.  This motion pulls the sand and silt down the drop-off, thus creating the underwater waterfall illusion.  Because waves constantly bring sand and weathering and erosion keep making sand and silt, is not very probable that this illusion will stop for a long time, if it stop at all.  I hope this was helpful.