yes, once you found all the star coins and spent all but 20, the last background(a screen shot from an older version) will be unlocked.
You can get them in red mushroom houses.
There is no invisible mushroom. The well-known reference is to Mario Brothers on Nintendo and somebody copied it to a (completely fictitious) cheat site post.
Mushroom house, life house and star house
Yes, every time you lose all of your lives and restart from your last "safe point" you can go back to the previous worlds and use the mushroom houses again
you have to get all the star coins and finished the game then go to the mini mushroom house.
There is no such item currently on Poptropica. This is an item and game function in "Mario Bros." and related Nintendo video games.
The green mushroom on the Super Mario Bros game is a 1-up mushroom. This mushroom gives an extra life to Mario. These mushrooms are rare to find in the game.
Throw a mushroom at Mario Throw a mushroom at Mario
There is no star mushroom in Super Mario Galaxy 2.
The mushroom that makes Mario bigger.
a mushroom for the mushroom kingdom
Mario is only 4 feet tall with the mushroom but 2 feet tall without the mushroom