I understand it's not the present tense, else it would be "goes". Is the sentence grammatically correct? If so, does it mean "you are seeing that it is going away"?
Verbs of perception (see, hear, notice, etc.) are followed by an unmarked infinitive — technically a present infinitive, but the past infinitive can't be used in this construction — or a present participle. I saw him go. I saw him going. The difference is a slight emphasis either on the action as a whole (infinitive) or as a progressive activity one observes (present participle). Though in the objective case, "him" is actually the subject of the infinitive "go." https://dictionary.cambridge.org/de/grammatik/britisch-grammatik/verb-patterns/hear-see-etc-object-infinitive-or-ing