what's this green stuff

[dbq5anlxj]

hello guys this is my friends tank. Its 75gallon and up running for 8 months already. His rock and sand covered by this green stuff. Can't blow it off It's pretty firm.Whats is this? thanks in advance

Alk 8.75
Cal 460
mag 1240
po 0
no3 10
temp 76.4
ph7.6-7.7 is this consider super low?
30% waterchange every two weeks

 

[blaxsun]

Algae. Yes, the pH is on the low side (along with magnesium). Zero phosphates could be a contributing factor with the algae. Boosting your cleanup crew (not all sure what you have) might help as well. Conches and nassarius snails have been fairly good for keeping my sand bed clean.