How do I make custom mushroom grow using bonemeal?

I wanted to make a custom mushroom that grows into a big one after I bonemeal it. 
Problem: the mushroom structure spawns not only when I click on the plant but also whenever I right-click on any block or in air with bonemeal in hand.

What am I doing wrong? how can i fix this problem?
Also how can I make custom grass block that doesn't generate underground?