If you are adding the right nutrients, NPK, GH, Traces, then light, all that is left is CO2.

DIY CO2?

Very likely the root of the issue.
Try adding more, redo the brew mix and see.

I'll bet if you add enough CO2, the moss will pearl, also, moss does better at cooler temps than higher, SG is not a cool place.
28C ought to be fine though.

Regards,
Tom Barr