Couple things to note;
If they are stretching up real tall, this means they arent getting enough light and are trying to get closer to it. This means you need more light.
If they start turning white they definately need more light. Mushrooms have microorganisms in them that use light to produce a food source for them and the mushroom itself. If there isnt enough light, they will starve... eventually starving out the mushroom. These organisms are what give the mushroom its color so natrually, if the mushroom starts looking pale, its food source is dying off. Again, like above, need more light.
If they start spreading out more, that means they are doing well. If you want them to spread, there are several ways to help this;
You can make them uncomfortable (ie: direct a powerhead at them) to make them move. Their foot will tear tiny bits off on the rock when they move and those bits will grow into new mushrooms.
You can cut them up in little bits (search the forum for proper techniques) and those will grow into new mushrooms.
You can rubber band it lightly to the rock its on and it will eventually split where the rubber band is pinching it.
Do a search in the forum for more detailed info if youre interested. Enjoy your mushrooms!