The 1849 LDS edition replaced something with somewhat, probably by accident; this secondary reading has continued in the LDS text. In the previous verse, there is an instance of somewhat, which seems to have prompted the change in verse 6:
Also notice that in both verses the word somewhat /something is followed by concerning.
Elsewhere in the text, there is one other occurrence of “tell somewhat”, but none of “tell something”:
Nonetheless, one can “tell a thing”, as in Helaman 14:9: “thus hath the Lord commanded me by his angel that I should come and tell this thing unto you”. Moreover, one can “tell things” (28 times in the text), for instance, nearby in Alma 54:7: “yea I would tell you these things if ye were capable of hearkening unto them”. The use of things in verse 7 is directly connected with the use of something here in verse 6: “behold I would tell you something concerning the justice of God”.
So there is really nothing inappropriate about something being told in Alma 54:6. In fact, the use of something implies that Moroni has a specific piece of information to convey to Ammoron. On the other hand, somewhat implies an impreciseness that is inappropriate for this letter of Moroni’s. But in Alma 56:2 (cited above), Helaman uses somewhat because he doesn’t intend to tell Moroni everything that has been going on in the war being waged in that other part of the land.
Summary: Restore the original something in Alma 54:6: “I would tell you something concerning the justice of God”; the secondary somewhat is actually inappropriate here.