Before buying moss check details such as amt.of lighting required, PH,KH,GH, temp.,co2,etc..
I have abt.16-17 types of mosses.
Personal experience willow moss was turning black, looking bad (dying) other mosses on the other hand were growing well. After checking on an int'l forum I came to know that this specific moss requires very cool temp.18-22 degrees can survive upto to 25 degrees n light needs 0.5 w/l minimum, CO2 n ferts are optional.
After doing the necessary changes (moved the moss into a planted aquarium with chiller temp.23-24 degrees, co2 n good lighting 0.8 w/l) within a week I saw +ve signs, green colored baby leaves, the moss came back to life n now its doing just fine.
Java n Flame moss are the strongest and quick growing mosses.