Much of that depends on your climate and watering habits. I could never just use perlite. It's too dry where I live and my plants would all die even being watered once daily (which I can't always do). Technically, throwing park on top of perlite should be fine. I throw a layer of Sphag on top of bark to hold in some moisture. You should do what works for your situation. There is no one single answer for an orchid in captivity...

Well said Suzabanana. I use clay pots with lava rock only. I water every day. That works for me considering my environment and my watering habits. Join an orchid society. They know the local considerations and will be glad to share their ideas. The green may be algae.

Zoe,

I agree with the above, if it ain't broke don't fix it, keep using the perlite if it is working. A bark cover is fine, I personally like the look of reptile bark as a cover as opposed to orchid bark, it is what I use for potted dirt plant mulch in the summer. You can find reptile bark where aquariums and such are sold for reptiles, it's at almost any store with pet supplies.

Good luck,

Bob

Thanks! I tried it with two orchids, I'll report back if anything weird happens :)