when to replace sphagnum on mounts/pots?
 

I've been keeping orchids in a mini terrarium for many months now, and the sphagnum on nearly all my mounts has been coated in algae. Many of the mounts and plants I've bought have algae-coated sphagnum so I know its pretty normal at least in greenhouses. On some of them, its a yellowish-green color which doesn't concern me.. Other mounts have sphagnum that's almost black and "crusty" looking. How do I know when the sphagnum is unhealthy for the plant?

The dark color is probably cyanobacteria, also known as blue-green algae. They need very wet conditions. Your terrarium may be too wet. As a bonus they fix nitrogen.

Oh wow! I found some pictures of cyanobacteria and that looks like exactly what I see on some of my mounts. A dark green slimy buildup. I have been worried that its been too wet, so I guess I'll try letting things dry down more between watering/misting.

Would there be a negative effect to such a build-up? I am assuming that any detriment it would have to the plant would be pretty apparent- roots die back, etc. Maybe I should just take the sphagnum off the most affected mounts to check?

I don't know for sure, but I would guess a dense mat of cyanobacteria could suffocate roots. In even wet habitats I've never seen orchids growing with cyanobacteria.