When mushrooms get overcrowded they will normally migrate on their own.
I had one small, flat chip of rock about 2"x3" that I bought with about 7 small mushrooms on it.
I placed it on top of a large piece of LR
and about a month later I went back to move the chip, and only 3 shrooms
were still attached to it.
All the others had moved just off the edge and attached to the LR
That's one of the great things about them that makes mushroom colonies easy to frag.
I just surround my large colony with rubble, and place one or two pieces of rubble in the middle of, or on top of the colony.
Within a few weeks at least one or two shrooms
have moved onto the rubble, and I'll remove those pieces and have a nice frag to trade or sell.
Sometimes you'll even see a mushroom that's detached and floating free in your tank who's "left home" to make a fresh start.
Don't be surprised if someday you move your rocks and find a hidden colony or two.