Any mosa spawning in an underwater cave is usually untameable, along with the other dinos spawning in that cave. Might just be in the Cavern if Lost Hope and the Cavern of Lost Faith, not entirely sure though. You can knock them out, but they won’t eat the kibble or meat put in their inventory. I found this out the hard way trying to tame a 180 mosa :(
How to get a high quality mosa saddle/mosa platform saddle on mobile:
Head into the yeti cave (tame one of the wolves in the cave to do this). In one of the big rooms there are multiple snow covered platforms. On one of the platforms is a red cave drop, and the loot of this drop includes mosa saddles and mosa platform saddles. I ran the cave ~100 times and I got 4 mosa platform saddles (1 journeyman 3 ascendent) and 2 mastercraft mosa saddles but I don’t even have a mosa lol.