Redish/Black growth on LR in nano tank

[kelley5454]

Ok so i htough all was going well all my critters are accounted for, I have beautiful clean rock where the cleaner crew has been and then we see this red and black growth on one of the rocks! Can someone help I dentify it?
My nano...
12g
10 lbs LR
2 Scarlet Hermits
2 Blue leg hermits
1 turbo snail
3 regular snails
2 Green Chromis
1 yellow tailed blue damsel
1 royal gramma
All water levels and temperature are good.

 

[mie]

How long has the tank been set up for and what does good water levels mean? it sounds like diatoms or some type of red slime alage, or it could be coraline alage

 

[kelley5454]

The tank has been set up for about 3-4 months.
by good water I guess I mean the following according to my test kit
Ammonia 0
Nitrite .25
Nitrate 0
Ph 7.8-8.0m( I added a bit if ph boost to it)
Salinity 1.025 (with a hydrometer)
temp 78
10% water change each week
Top of with fresh as needed