I suppose this would depend on . . . average enemy health. Lichbane (in your build) is worth about 500 bonus damage (before reductions) every 2 seconds ('cause Mord isn't a "1-skill toon"), while Liandry's is worth (at best) ~50% of their health, assuming nothing overrides Mace of Spades (and/or that CotG procs as a non-DoT), and that you spam Q, and they're slowed/stunned/snared for the full duration (20 seconds). Getting that 40% requires that, while the target is "mobility-impaired," you hit them with each spell in succession (getting Q on them as a solo proc), spacing them all 3 seconds apart, and getting a 2nd & 3rd Q in during CotD's duration.
So over this hypothetical 20 second interval, you could either do ~500 x 10 = ~5,000 damage, or ~50% of the enemy's HP; 5k hp / 0.5 = 10k, so when the average enemy hp is (effectively) 10k+, Liandry's will be better. Note that this applies to 5000 hp tanks w/ 100 MR as well as 2000 hp mages w/ 400 MR, and everything in between.
Because of how Mord tends to work, and the woeful lack of lasting CC, Liandry's will be less efficient on Mord; he is more likely to get 15-20% out of Liandry's (at best), and the lower the %, the more hp/mr they have to have (on average) to make it more effective. Liandry's works best (and was designed for) champions who prefer to "lane," such as Vladimir.
EDIT: I just realized that Liandry's is base 5% current (rather than max), which means that all of those best-case percentages have to be halved again. Bottom line: Lichbane is still better than Liandry's.