alpha bibliography style is one of the original styles created for BibTeX, i.e., it's been around for more than 20 years. As such, it does not recognize (i.e., process) fields such as
urldate. You need to use a more modern style file.
The IEEE provides the style file IEEEtran.bst as well as several related style files; one of these files may meet your needs. Peruse the user guide, How to use the IEEEtran BibTeX Style, for more information.
Specifically, this style file provides for an
entry (something in BibTeX that starts with an
electronic. The user guide has the following to say about this entry type:
IEEEtran.bst provides the “electronic” entry type for internet
references. IEEEtran.bst also provides the aliases
“online”, “internet”, “webpage” and “www” for compatibility
with some existing BIBTEX database and style files. However,
“electronic” (or perhaps “online” for those who also use
Philipp Lehman’s biblatex) should be used for all new
work. IEEE formats electronic references differently by not
using italics or quotes and separating fields with periods rather
than commas. Also, the
date is enclosed within parentheses
and is placed closer to the title. This is probably done to
emphasize that electronic references may not remain valid on
the rapidly changing internet. Note also the liberal use of the
howpublished field to describe the form or category of the
address fields may also be used.
From this, it would appear that you could rename the
urldate field name to
date and be all set.