acme : mace :: alga : ?

Let's track the mapping arrangement of the letter positions from the first word to the second word:
Word 1: a c m e (assigned with indices $1,2,3,4$)
Word 2: m a c e (arranged in indices order $3,1,2,4$)

Now apply this exact position rearrangement sequence ($3,1,2,4$) to the letters of the word a l g a:

* Position 3: g
* Position 1: a
* Position 2: l
* Position 4: a

Combining them yields g a l a.