I think "digit to the right of the decimal separator" may achieve nearly the best, if not the best, balance of conciseness, formal correctness, and lack of ambiguity among all the ways you can describe such a digit.
You could say "fractional digits" and (probably) be correctly understood,
at least in context. For example, see How to extract fractional digits of double/BigDecimal
or How many fractional digits do I need to represent a number of base $m$ in base $n$?
The digits to the right of the decimal separator are sometimes called
"the decimal digits" of a number.
This term is defined in this way by various sources for whose authenticity
I cannot vouch, such as
this
or this.
Documentation for databases sometimes uses the term "decimal digits"
to refer to the number of digits to the right of a decimal separator
(for example, here
or here),
but the phrasing is such that they appear to be using "decimal digits"
as a shorthand for "number of decimal digits," implying that each of the
digits to the right of the decimal separator is an individual
decimal digit.
I think the term "decimal digit" is ambiguous, however, since one sometimes sees statements such as that the decimal digits are $0,1,2,3,4,5,6,7,8,9,$
or that the number $2^{32}$ has nine decimal digits.
That is, in some contexts a decimal digit can refer to any digit that appears or might appear anywhere in a decimal representation of a number.
Worse still, the asker of
one of the questions mentioned above
used the term "decimal digits" to refer only to the digits to the left
of the decimal separator.
