Does touching the anus during prayer, in order to alleviate the fear of invalidating one's ablution, constitute an excessive movement that invalidates the prayer?

It is advisable to avoid intrusive thoughts (waswas) and not to dwell on them. The slight movement mentioned does not affect the validity of the prayer. It is important to note that touching the anus without a barrier is considered among the nullifiers of ablution (wudu) according to many scholars.