sure to grow block question.

ok so im using a ebb and flow system... i got 5 plants growing in 4 inch sure to grow blocks.... first time i tried the sure to grow system.. supposibly they are ph neutral and algae doesnt grow on them... but after 9 days one of the blocks has some green patches starting to develop... only 1 out of my 5 of my plants is doing this... anyone have any experience with sure to grow? could this actually indeed be algae?

p.s. my ph is at 6.5........ if this is algae how do i handle it with sure to grow blocks?

thanks guys