Does the prayer become invalid if the worshiper makes a mistake in reciting Surat Al-Fatihah and pronounces the word "العالمين" (al-ʿālamīn) with a sukoon on the lām before the mīm?

The prayer of someone who makes a mistake in Al-Fatiha out of forgetfulness is valid, whether or not it changes the meaning. This is because it is akin to someone who speaks during prayer out of forgetfulness, and that does not invalidate it. A melodic error (lahn) that changes the meaning, if due to ignorance, forgetfulness, or an infirmity, does not invalidate the prayer.