Algae in Phal's clear pot

[Val]

Hi everybody,

I'm relatively new to orchids (6 months and 37 orchids). Although Phals have already begun to "speak to me", the algae are uninvited guests. I have noticed algae inside a couple of pots and I'm wondering:

1. Could algae be detrimental in any way to the plants?
2. What does the presence of algae signify? (Like to much / not enough light/humidity/warmth/watering/etc.)
3. Is there a way to eliminate algae without harming the orchid?
4. Are algae prone to expanding to neighbour pots?

And a somehow related question: doen anybody know an European equivalent of Physan 20?

MTIA