An object is normally a noun. But a noun can be replaced by a pronoun (him/her/someone etc) or by a gerund. Your sentence contains two verb forms "cannot" and the infinitive passive "be seen". You can describe such a structure simply as verb + infinitive or, as some grammars do, say that the infinitive is object to can/cannot. As an infinitive can be seen as a verb form with noun character this view is not impossible. But it would be better to talk of infinitive in object position to have a precise description of the structure.
For learners I think it is best and simplest to see such structures as verb + infinitive, as there are hundreds of verbs that can be followed by an infinitive.