Coraline on glass but not on rock

[argileh]

I have a descent coraline algae growth on the glass, but almost none on the rocks, and during the past several weeks there is macro algae (brown, bush like) growing out of the rocks. Could this be an indication that the nutrient level is high.

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it's a 29g boceanic bio-cube with top removed (suspended MH), skimerless, but with 15% weekly water change. fishes are one small perc clown, one small pygmy angel and yellow watchman goby.
forgspawn, tocrh and xenia colonies are doing fine. Green candy cane has not expanded in a while

Water parameters are:

- Nitrates: approx. 12 ppm
- Salinitiy: 1.026
- PH: 8.3
- Calc.: 420 ppm
- Alk : 9 dkh (salifert kit, not sure if the kit could be one of the defective ones)

Lights are 175MH 10000k that are about 13 months old. Not sure if this is a factor, but any way I am expecting an Iwasaki 175w 15000k in few days.

Any opinions on the lack of coraline on the rocks, as well as the unsightly macro algae growth