A VERY WARM WELCOME to MASA, Roo! Great to have you on-board! Would you please be so kind, as to create a new thread, under our "New Members" forum, and tell us more about yourself, your tank/s and your reefkeeping experience?
To answer your question - yes - diatoms do live off silicates..... All new water and new sand have diatoms that starts to grow off them....
Can you please elaborate on how long have you had your tank set up for?
Also - please be patient - once the silicates in the tanks and substrate have been "used up", the diatoms also disappear.....
Good luck mate! Hope you won't be a stranger now! Hope you enjoy our company! It's an amazing hobby, and life changing experience!