Section 230 (of Title 47) grants immunity from civil liability under certain circumstances, so the website is not liable to the porn-distributor for taking down their images. There are also criminal laws pertaining to child porn, such as 18 USC 2252. It is not a crime to "see" child porn. Instead, the crime is defined with reference to one who
knowingly receives, or distributes, any visual depiction...
of child porn. Therefore you cannot be prosecuted if you do not know that the image is has the prohibited characteristics (is child porn). See US v. X-Citement Video, 513 U.S. 64 for discussion of th scienter requirement. The surrounding circumstances (the report) at most indicate that the moderator has some reason to believe that it is porn, and the immediate deletion of the material supports the conclusion that the moderator's action complies with the law.
Paragraph (c) of that law also provides a defense, in case of prosecution:
It shall be an affirmative defense to a charge of violating paragraph
(4) of subsection (a) that the defendant— (1) possessed less than
three matters containing any visual depiction proscribed by that
paragraph; and (2) promptly and in good faith, and without retaining
or allowing any person, other than a law enforcement agency, to access
any visual depiction or copy thereof— (A) took reasonable steps to
destroy each such visual depiction; or (B) reported the matter to a
law enforcement agency and afforded that agency access to each such