I was once masturbating and a white (with a pinch of grey) fluid came out of the tip of my penis. It only happened once. Additional information: I am 12 years old and I am on stage 2.7 with puberty. The fluid thing happened only three months ago.
The fluid was semen, but your body hasn't matured enough to produce it in any quantity yet. That will eventually come later.
While I know it feels good to play with your genitals, you shouldn't be making a habit of it. In the not so distant future there will come a time when your body is producing semen and you'll feel the need to get rid of the excess. Masturbation is one option for handing that need so long as you aren't using pornography or lustful thoughts to get yourself sexually excited. But until that time chasing after pleasure for pleasure's sake is too wide open a door to let temptation and sin into your life.
Thank you very much. You have been of a great assistance to me and many more boys who have questions about puberty.