Suggest you isolate the above plant from your other normal (i.e. healthy) plants now.
Leave the above plant alone (i.e. please do not water it and do not feed it anymore fertlizers) at a shaded and cool spot with very good air movement.
The yellow leaf cannot be saved and will drop off soon. If the plant starts to grow new roots (i.e. the plant can be saved) about 1 to 2 weeks later , suggest you replace all the sphagmum moss (too much algae) quickly.
I guess so. Although I don't know what plant it is, I think it does not like to be so wet as nearly all the roots have turned bad. In my collection of Phalaenopsis, only the roots of certain species can withstand such a wet condition without becoming bad.