Are the intermittent brown drops that appear after the fortieth day considered menstrual blood, invalidating fasting and prayer?

These brownish discharges (al-kudrah) are not considered menstruation if they do not occur during your regular period, and your fasting and all other acts of worship are valid. You must perform istinja' (washing the private parts) and wudu' (ablution) for prayer due to this kudrah. Even if it occurs during your regular period, it is not considered menstruation unless its total duration reaches a day and a night, which is the minimum duration of menstruation according to the majority of scholars.