What are the main formatting differences between ...

Basically the academic paper is a formal document formatted by rigid specification that was drawn up by an official organization, whereas a memo is an informal document that doesn't have to meet the requirements of a rigid specification. The rigid academic specification specifies font style and size for body text and footnotes. It defines size page, margins and gutter.

The memo does not have to have a specific font and can even be handwritten. 

An academic paper must follow many many rules to be considered acceptable and legitimate.

One of the biggest things to consider when writing an academic paper - aside from correct grammar, spelling, punctuation, etc. - are the citations. See here for an MLA guide to citing.

A memo is closer to letter-writing, which, while not quite informal, is also not as subject to rigid rules of academic writing.

How to respond in the form of a memo?