That might be the beginning of Blue-green algae.

I would use Cutrine Plus at a 0.2 ppm dose.

Here's the label so you can figure out how much to use:
Cutrine Plus Liquid label


www.hoosierpondpros.com


http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).