Confused / worried last year did an mri non contrast incidental arachnoid cyst found 30 mm * 18 mm without compressing brain, this year ct contrast for non-specific symptoms arachnoid cyst 10 mm "thick" and is compressing brain? is this growth or?

Not clearcut: Provided data is confusing, and it is tough to tell the impact of the cyst. You truly need to talk to both radiologist and your doctor, and have them demonstrate the "brain compression" and correlate to your current symptoms. You perhaps will ultimately benefit from a second opinion from a neurosurgeon. Key is the dynamic effect exerted by what may be a change.