What is this spider web type algae? Help!

[kgstep]

My tank is about 6 months old. I do weekly water changes, and test my water weekly, my corals have been growing and happy. I went away for the weekend and I came home and it's like they all flipped. They are all closed up and look sad, Ive since lost 3 corals and I don't want to lose anymore. There is this long dark algae that just binds up everything, I've been sucking it up with my turkey baster, I e done a water change I'm not sure what else I should be doing to fix this.

Water test results:
PH. 8.4
Ammonia 0
Nitrite 0
Nitrate 5 ppm
Phosphate .25
Kh 161
Salinity 1.026
Calcium 360(I know it's low)

 

[Dan_P]

My tank is about 6 months old. I do weekly water changes, and test my water weekly, my corals have been growing and happy. I went away for the weekend and I came home and it's like they all flipped. They are all closed up and look sad, Ive since lost 3 corals and I don't want to lose anymore. There is this long dark algae that just binds up everything, I've been sucking it up with my turkey baster, I e done a water change I'm not sure what else I should be doing to fix this.

Water test results:
PH. 8.4
Ammonia 0
Nitrite 0
Nitrate 5 ppm
Phosphate .25
Kh 161
Salinity 1.026
Calcium 360(I know it's low)

Looks like cyanobacteria