1. "not equal" is written # in TLA+ (or /=), "~" is Boolean negation.
2. I believe "->" represents implication in your formula, but it is written "=>" in TLA+.
3. The expressions on both sides of an implication should be Boolean, but the _expression_ on the right-hand side is y-1.
4. The (action) formula corresponding to your informal statement can be written as
\/ y' = 0
\/ y # 0 /\ y' = y-1
A similar formula (but with a slightly different meaning) is
y' = IF y # 0 THEN y-1 ELSE 0
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