What is this build up?

[WIReefer]

Have a frag tank in the house with some live rock it’s fully cycled and been running for about 4 months have some coral in the tank was just curious what is this build up on the bottom of tank do I need more flow? Just added a snail and crab to see if they eat it. But it’s quit a bit.

 

[WIReefer]

Someone told me to add a yellow tang once to the tank so you agree?