Donut coral question

[Ruth]

I have had a donut coral for about 8 or 9 months. When I first got it the color was bleached out a bit and was kind of light lime green and pink. I put it on the sandbed of my 230 and over the next few months it did very well - the coral grew, was constantly expanded, and the color changed to a darker green and more of a salmon color. I noticed that about a month ago it was no longer expanding very much and didn't know what was going on until I saw my hippo tank picking at it. I kept watching and sure enough he was there everytime this coral tried to expand. I removed the coral and put it in my 24g nano. For the first week it was fine - expanding again and color good. About a week ago I noticed long black strings coming out of one of it's mouths - a lot of them. I thought the coral was a goner but as soon as it expelled these strings it seems to be just fine. Fully expanded - color good - mouths closed except when eating and feeder tentacles coming out at night. Any ideas? What were those long black threads?