Within Org SRC_BLOCK blocks, LaTeX inputs are not interpreted nor converted to symbols on export. You can tweak the export, though, to achieve what you want:
The following filter replaces
\lambda by their equivalent Unicode symbol:
(defun my-lambda-to-unicode (output backend info)
"Convert lambda to Unicode on export to html."
(when (and (eq backend 'html)
(string-match "\\\\lambda" output))
(replace-match "λ" nil nil output)))
A simpler workaround, though, consists in entering the equivalent Unicode symbol directly into the buffer. To make the process easier you can use
abbrev-mode, and have lambda be converted to the Unicode small greek lambda automatically. Here's a tutorial on how to deal with Unicode in Emacs.
An alternative is to use yasnippet and have snippets to insert symbols. The abbrev seems more efficient, though.