grape Caulerpa dying fast!!

[SeaMonster]

Not sure why but first they develop visible black lines on each grape, the next day they are dead. they turn white and semi-transparent.

big 3 are 0, calc 450, alk 280 (using a dip test)

corals, fish, and chaeto are fine!
I just removed about 90% of my grape my tang will miss it

Any ideas?

Thanks

 

[91YOTA]

count yourself lucky... i had that stuff once... always tried to get rid of it with no luck