There are 2 solutions for your question.
Update AUCTeX to latest version 11.90. I suggest you do it via ELPA as described in the manual. If you do so, please also read Quick Start. And definitely put this in your init file:
(setq TeX-parse-self t)
Quick fix solution
elsarticle.cls internally loads
graphicx and some other packages. AUCTeX does not parse the .cls file to be aware of this. You can give AUCTeX a hint by loading these packages again in the preamble of your .tex file. With
TeX-parse-self set to
t, AUCTeX loads its support files (jargon: style files) for these packages and your file will look like this:
Also note that now you get auto-completion support for macros provided by
natbib when you hit C-c C-m citep RET. Things get even better if you use RefTeX.
This includes writing an AUCTeX support file
elsarticle.el. For what you're asking, the following lines will be sufficient:
;;; elsarticle.el --- AUCTeX style for `elsarticle.cls'
(TeX-run-style-hooks "natbib" "graphicx" "geometry")))
Customize the variable
TeX-style-private to a directory of your choice, e.g.
(setq TeX-style-private (expand-file-name "~/.emacs.d/auctex-styles"))
elsarticle.el in that directory, restart Emacs and load your .tex file. This should then work without reloading
graphicx in your preamble.