I suspected it might come from that, the thing is I tried long and hard to get my MC to carpet, and the water column is like 7G of real volume, so idk how to vacuum properly with such low volume / without unrooting MC.See all the brown silt? THAT is feeding the cyano. Light? Every aquarium is bathed in light. You need light.
So clean that substrate - not just once- with every water change,clean your filter with every water change..and make weekly water changes.