Orchid medium problems

[beckky]

Hello, I am back with another question and a very loooooong description
Some months ago I bought 3 orchids (phalaenopsis) from a local nursery. They looked amazing, one had 3 flower spikes with huge flowers. I was watering them once a week (they were potted in pine bark, I believe). Eventually they lost their flowers and I cut the flower spikes. Everything was going well until one of them started to get one yellow leaf, and then another and both of them fell. More leaves had the same issue and at some point she only had 3 remaining leaves and the stem was really dark. One day I wake up and found that the crown with 2 leaves just fell. It was the end for her.
(One thing that I noticed with these 3 orchids was that the roots never turned gray, they were always bright green, even when I didn’t water them for 3 weeks.)
I started to inspect the other two orchids and found that one had some sort of mold going up the stem and one leaf was already on the path to turn yellow. I decided to take it out from the pot and inspect the roots and that is when I found out that she still had her seed plug attached and all the roots in the plug or around were totally rot. I cleaned everything, I cut the black and mushy roots and left it without any potting medium. I also took the other orchid from the pot (she didn’t show any sign of sickness except permanent green roots) and found that she also had the plug and some rotten roots.
At this point I went to the nursery and took the orchid with me. The lady was very surprised to see the plug but thought it was ok ( I argued that it didn’t seemed healthy to have that old thing there and that the bark and the plug would dry out at different speeds and that could lead to root rot). After seeing the photos of the dead orchid she decided to offer me a new one ( a very healthy one, she said).
So.... now I have a new orchid, I haven’t water it since I brought her home (10 days ago), her roots are super bright green, it looks like there is some sort of green moss or algae in the pot and the bark in the surface has white specks and white areas that for me look like mold.
I have no idea what to do.
I ordered some good potting medium for the 2 orchids that had the plug, but I don’t know what to do with this new orchid (she is in bloom).
I will attach some photos to illustrate the whole situation.