Once again, Watts per Gallon is meaningless. Tell us what kind of lights (I am assuming PC) what color temp the bulbs are, how high the bulbs are off the water, how deep the tank is and we can advise as to the adequecy of your lighting.
I assume you have a Standard 55G, which is 18" high, and that you are using 2 daylight 55W PC bulbs over your tank. In this configuration, many mushrooms will be prolific when placed about mid-way up the tank (about 10" from the bulbs)
I had the same setup and successfully kept 4 or 5 different varieties of mushrooms. Be aware that there is a 65W bulb that fits your fixture (same length, bigger diameter) that works with most ballasts. I would recommend that when you replace your bulbs you use the 65W bulbs. Also, if you were to add 2 more 55W (or 65W bulbs) you could keep a lot more corals.
Ernie