If it hasn’t already been suggested, a word that spells a different word when read backwards should be a margana.
Bacrostic?
There doesn’t appear to be a generally accepted term, though some people have coined words such as “semordnalip” for them.
“Semordnalip” seems to be the only one with any presence, silly as it is.
Words do not have the property of being anagrams generally. Being an anagram is a relational property, e.g. “star” is an anagram of “rats.”
Palindromes are not relational properties. ‘Madam’ is a palindrome full-stop. ‘Madam’ is not a palindrome of ‘x.’ I think we want the word to describe the property of ‘being another word when read backwards’ with a non-relational phrase (i.e. one that doesn’t take on an object).
“Anagram” is not really a property, but a type of relation (like ‘being above’).
I will take the category error.
But I don’t think we are dealing with a category error. A word, when spelled backward is another word. That other word, when spelled backward, is the first word.
This is a specific type of anagram relationship, between two words. It is not a property of either word. The name would apply equally well to both words in question. The manner of phrasing may have suggested that the term was to be applied to “a word”, as a singular entity, like “madam” as a palindrome, But in this case the “a word” was used in a general sense, as in “what can we call any word that when spelled backwards makes another word?”
“Rats” and “star” are both this type of word. They are anagrams of each other that require no changes to the letters immediately to the right or left of any of the letters in either word, unless the letter is the first or last, in which case it is the letter to the right, if the first letter, or left, if the last letter.
It’s also the most obvious anagram of either word. Maybe “obagram”.