Hello Jule,
Glad you got this wonderful species! This species needs heat, full or near full sun, stagnant air, and plenty of humidity. Where you stick the plant in these conditions doesn't matter too much...
As for calcium for ghost orchid, I have not heard of this. Instead, I have heard of occasional banana feedings for them, as the growers also have Staghorn ferns (Platycerium sp.). Water rich in algae or just any water is good enough for this species; RO or distilled water will show no differences. For watering, try and water heavily during the summer months (keeping it wet at almost all times except for night), and during the winter months, keep the medium damp. When the plant gets mature, it can stay on the twig to grow, or placed on a larger mount, or put in a wood slat with sphagnum moss (which will retain nice amounts of moisture).
As for any mineral build up on roots from tap water, I have never observed this. In a single location here in Sarasota County, this species grows JUST above the hide tide level, perhaps getting some of the spray from the salt water. This species is more sensitive to light levels than anything else. Temperature isn't as important, as temps. in situ here in FL vary in the winter months without affecting the plants. Also, humidity can be replaced with a medium that retains an adequate amount of moisture (available moisture).
I hope this helps, hopefully your conditions will be suitable for this wonderful species!
-Pat