One of the prerequisites for diatom growth is nutrient content.Pretty much the main one.
Phosphates and silicates are the main ones involved.Any place those nutrients are concentrated will start diatom growth regardless of light level,just that usually in a higher light area they are outcompeted for the nutrients by algae.
It's possible that where the crushed coral is in direct sunlight it is leaching its mineral content faster. Since phosphates and silicates are capable of stratifying there's a good chance that that is a "dead zone" with little circulation.
Have you tried changing flow through that area by changing filter returns around or with a small powerhead ?
Also a good chance that this being a new setup with a recent reseal the problem will go away (in time) as the nutrients are used up.
I'm curious on the choice of crushed coral as part of the substrate. Are you trying to buffer pH higher ? If so,why ?
Far as something eating it,any fish that would touch it would be small enough to be O-Chow.That leaves snails...most of which that would actively prefer it (pond snails,some Ramshorn) are also O-Chow...which are "iffy" on it if something they like better is about. Apple Snail and Malaysian Trapdoor are large/tough enough around Os,but diatoms wouldn't be first choice on their menu.Some crustaceans....but crustaceans are way high on an O's menu.
It's better to find the cause than to make additions to band-aide it.
Just think about it.