Peter,
The London Guildhall Museum Library holds copies of the Lloyd's Captains Register, which lists alphabetically masters and mates who received their certificates between 1851 and 1869. Copies of entries between 1870 and 1948 are held at the London Metropolitan Archive (as a start, see
here).
Information listed includes dates of qualification and names of vessels in which they served.
If you scroll down the page you will see the entry for "Marsden - Myles", which will include listings for Captains with the surname Moss.
Help for those wishing to use the Archive site and the various search options available are given on the site. Hope this helps.