The given expression is:

$18 - y^2 + 12$

To identify the coefficients, let's break it down:

1. The term $18$ is a constant and does not have a variable, so it doesn't have a coefficient.
2. The term $-y^2$ has $y^2$ as the variable, and its coefficient is $-1$.
3. The term $12$ is also a constant, similar to $18$.

So, the only coefficient in this expression is $-1$ for the $y^2$ term.

Would you like any further clarification on this?