Well I'm going to throw you a curve ball now sorry, this isn't true, there is an area around corals, especially coral colonies called the boundary layer. This area is the most diverse microbial ecosystem on our planet. The inhabitants of the boundary layer have a direct impact on the health and well being of a coral. Corals can manipulate bacteria in this layer to such a degree that on release of certain hormones they can alter a bacteria at the gentetic level . Essentially coral farm these bacteria and other microbes to derive various benefits. This thin sliver of water between a corals mucus layer and the water column is part of the complex ecosystem. There will be a boundary layer even in captive reefs because corals have evolved this as a mechanism for life. So beneficial bacteria do not just live on surfaces in our systems. Many don't know this so hopefully after reading this you might also share it with others. Feel free to research this phenomenon yourself too, it's very well scientifically documented.