The algae does look nasty and keep coming back every year, but so far I have not seen them cause any problems (at least I think I do more harm to my plants than the algae).
I use two methods when dealing with algae growth on my SH pots depending on the situation.
Option 1: Transfer the plants to another SH pot and sterilize the old pot.
If the plants are sturdy enough for relocating to another pot:
1. Get a similar sized SH pot
2. Remove the plant and rinse the pellets from the old pot.
3. Transfer the plant and pellets to the new pot.
4. Let the old pot dry out.
5. Once the algae dries up on the old pot, just rinse it with a strong water flow and it should easily fall off.
6. Sterilize and store the cleaned up pot for later use.
With this method you rotate two SH pots for one plant as often as needed.
Option 2: Use Physan 20 to kill the algae.
1. Mix an appropriate solution of Physan 20 and water.
2. Cover the reservoir holes to prevent the physan from flowing out.
3. Pour in the solution into the SH pot all the way up to the level of the algae growth.
4. Let it sit for 10 minutes and then uncover the reservoir holes. The Physan 20 solution will drain all the way down to the reservoir level.
5. After 2-3 hours, pour generous amounts of water into the SH pot to rinse the reservoir. Physan 20 can kill algae in as little as 20 minutes. You will want to remove as much dead algae from the reservoir as possible. Live algae is relatively harmless to the orchid - dead algae fouls up the water and could cause problems for the orchids.
This method allows you to keep the plant in the same pot, but it is a little more tedious and some algae strains might need a stronger or longer dose of Physan 20 to kill. I do not know the effects of stronger concentrations of Physan 20 on orchids so be careful there.
A periodic spraying of Physan 20 into the SH pot goes a long way in preventing serious algae buildup in the first place.