Does the presence of rheum around or on the eye after Fajr prayer prevent water from reaching [the skin], and if so, is it obligatory to repeat the prayer?

The rheum (ghamis), which prevents water from reaching the skin, invalidates the ablution. If it is possible that the rheum appeared after the ablution, then the Fajr prayer does not need to be repeated. But if that is not possible, then it is obligatory to remove it and repeat the prayer, because one of the conditions for the validity of ablution is for water to reach the skin. And one's duty is not discharged except with certainty. Ibn Taymiyyah holds the view that a minor barrier is excused.