In the case of the photos above, what you are seing are the coral's spicules. These are usually needle shaped but sometimes come in other shapes. These are what give a soft coral it's support, since soft corals do not have a coral skeleton like hard corals do.
What your seeing is that the coral is damaged. This can come from various sources. You livestock might be eating the corals. There may be a bacteria infection in the coral. The coral could have suffered physical damage. It's also possible low water quality is a big factor in the problem.
This brings up the question, what fish do you have in the system? Many consider soft corals to be a fine meal, especially the large angles and butterflies.
Getting back to the coral. The white area on the coral is likely dead. The really big problems comes if you see any black area. This is an area where the coral is decaying and on it's way out. To further test the coral you can give it the smell test. A soft coral will smell bad, but one on the way out really smells foul and nasty.
If this is a single coral and all other livestock is doing well, and there are no black areas, the coral might recover. It's usually a good idea to make sure the damaged area gets some good water flow. Sometimes the coral will heal on it's own.
If you see black areas or the coral keeps getting worse, frag off the good areas and attach them to new live rock. Discard the bad areas. That way you at least save part of the coral.
You coral seems to have a fairly large area that has gone bad. you maight want to frag off some pieces of it anyway just to be sure you have something.
As a note, when you talk about water parameters, please post exact numbers for what you tested. Just telling us they are ok does not give us any information. What seems ok to you might indicate a problem to someone else.