SPS problem..please help me identify.

[jimbo222]

Please help me identify this flesh problem on my sps.
I noticed it yesterday and took a photo.
Today i took another photo you can see how fast the flesh is coming off.
I watched it yesterday night and noticed two pea size crabs on it .
I.v had it for over a month now and it has shown great signs of growth and colour until yesterday...non of my other sps's show any sign/s of problem.
all water paremeters where checked ok..
Any idea's would be appreciated. thanks