What is the fluid that exits a girl when she touches her friend or when she contemplates, and is it semen that necessitates ghusl or affects the validity of prayer?

Merely touching a female friend or a fleeting thought does not usually necessitate the surging of desire (shahwa) or the emission of semen (mani) or prostatic fluid (madhy). The discharge you find might be the normal secretions known as vaginal moisture. Ritual bathing (ghusl) is only required upon certain conviction of the emission of semen. If in doubt, one may choose whichever ruling they prefer. As for desire (shahwa), it is a perceptible matter that occurs with thoughts of intercourse or its inducements, and its abatement is its disappearance and the subsiding of its intensity.