SPS turning white.

[Delphinus]

Without a picture here's what it could be.

- Bleaching. "Bleaching" refers to tissue that turns white, but is otherwise "still there." It can lead into RTN, or STN, but sometimes can come back. It can be caused by many things including light shock, or shading. If the top of the coral shades the bottom, this can happen, but it's usually with larger colonies.
- RTN or rapid tissue necrosis. The coral looks white but it's not bleaching - it's the skeleton. The tissue itself is gone, melted. Many causes. More on this later.
- STN or slow tissue necrosis. Same as RTN but much slower recession. Again, many causes.

Bleached tissue can come back (no guarantees of course). With RTN and STN, it's gone. You have to address the cause and hope that there's enough coral for it to grow back. With RTN the coral is usually a total loss.

Ok, causes. Ultimately, it's a reaction to stress. This could be low Alk, excessively high Alk, low Ca, excessively high Ca, Ca/Alk out of balance, presence of NO3, presence of PO4. Can also be allelopathy (chemical warfare), not enough light, too much light, too hot, too cold, too much temperature swing between day and night, too much pH swing, fish that nip, crabs or other inverts that nip, moon phases, you looked at it wrong and whether the day of the week it is currently ends in "y". (Welcome to the wonderful world of SPS. ) Sorry I realize this isn't helpful but the problem is SPS are like the canary in a mine sometimes. And although some are really hardy and can weather things, others are not so much.

Edit: I forgot to mention that sometimes RTN is protozoan in nature - meaning it's a parasitic infection of an organism. And likely contagious. This is a risk with wild-collected SPS.

One thing that we can likely rule out at this point is "excessively high Alk". For one, unless you're dosing without testing, the likelihood of this is low to begin with. But also, "alkalinity burn" tends to be more systemic in nature and literally looks like the coral is burnt.

SPS are a tricky mistress. I like to think of them this way: there really is no such thing as "stasis" or "stability". Ie., they are either growing or receding. So you pretty much need to stack the odds in the favour of growth and that is through good lighting, good water flow, low-to-zero NO3, zero PO4, good Ca and Alk numbers (either through dosing or reactors).