Does the only difference is "being continuous"?
what is the difference in terms of meaning?
... with a symbol used to express ... .
... with a symbol being used to express ... .
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"used to express" coming directly after the noun symbol could refer to the symbol's typical role, the role we see it in most often.
"being used to express" could refer to an ad hoc or a particular application of the symbol, to express some idea in a specific context, rather than in general.
The symbol used to express a warning to the user in that GUI is an exclamation point inside a triangle.
A symbol being used to express the idea of poison to a young child should invoke fear and disgust.