My coral is angry?

[Moogled]

It's probably either releasing seed or expelling waste/defense mechanisms. Some people prop their plate corals on rocks, but it is usually best to keep them on the substrate.

What kind of plate coral are you keeping? The short tentacled Fungias or long tentacled?

[Peterupnorth]

From pics on a website, I'd say its the long tentacled.

[Moogled]

Expelling waste/toxins. Either one is possible then.

Keep an eye out and see if any other corals are affected to avoid chemical warfare.