when to replace sphagnum on mounts/pots?

[itzi]

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?