You should have plenty of lighting to keep mushrooms and zoos. Most softies are fine under PC lighting. Try placing it a bit higher and if you aren't already, use an additive to promote the best health possible, like Coral Accel three times a week or Coral Vital after every water change. Keep your calcium levels up. I learned the same way you are, anything under MH will look completely different under PC, and vice versa. This isn't to say you won't get the maximum health out of the coral, it's just that it will take a while to adjust to the lower lighting and schedule. Many of my mushrooms were smaller and not as "umbrella" shaped at first, but after a month they began to swell up and took on a new life. They most likely need time to adjust from the MH. Be patient and see if things look up for it!
Oh, P.S. mushrooms often look like a cup or trumpet when feeding. When they're "sun bathing" they'll spread out flat. HTH! h*