From Wikipedia's definition of algebraic expression
In mathematics, an algebraic expression is an expression built up from integer constants, variables, and the algebraic operations (addition, subtraction, multiplication, division and exponentiation by an exponent that is a rational number).
$4.5$ in $x - 4.5$ is not an integer, so it would be algebraic or not?
$$X - \frac{9}{2} = \frac{2X - 9}{2}$$
so according to the definition, your expression is indeed an algebraic expression.