EVERYTHING* needs phosphorus, and inorganic phosphate is the easiest way to get it for corals.
Almost everything uses Adenosine Triphosphate (ATP) as an energy carrier - removing (ADP) and then adding back a phosphate ion to move around an electron. And everything* uses phosphorus as the backbone of it's DNA/RNA.
There's some research showing high phosphate slows growth in corals. There's conflicting research showing that given available nitrogen, it actually increases growth. We're not really sure. What we do know is that phosphorus deficiency kills corals rapidly. A little high is much safer than a little low.
If your lights aren't on, you don't need to be worrying about algae. And when your lights do go on, algae is much preferable to the things that survive in phosphate deficient environments (dinoflaggelates, etc)
(*we're aware of one bacteria in one lake in California that uses arsenic instead - although it much prefers phosphorus)