The SNP accused the Scottish Conservative leader of "brazen hypocrisy" towards his constituents and the wider public.
Mr Ross previously said he was "extremely sorry" for the "bad error", adding: “It was all of my own making and I tried to rectify it as soon as it came to light."
The Scottish Tory leader is the MP for Moray and has been a list MSP for the Highlands and Islands since May.
Ms Adam, who represents Banffshire and Buchan Coast, wrote: "I am writing to you to express my concern at the brazen hypocrisy you have shown over the past few weeks towards your constituents and the people of Scotland and why you should consider your positions as both an MP and MSP and decide which of these jobs you value most."
She added: "It was revealed in the press that you ‘forgot’ to declare £30,000 worth of earnings to the House of Commons.
"While most people find this completely ridiculous, it is further compounded when you consider that people, some of whom will be our constituents, are cruelly punished by your party for missing appointments with the DWP [Department for Work and Pensions], despite presenting legitimate reasons for absence.
"It appears that you hold those on Universal Credit to a higher standard than yourself in this regard, supporting the cruel sanctions regime they are subject to for their completely valid justification for missing appointments with the DWP."
Ms Adam continued: "Ultimately, if you consider your MSP salary to be so unimportant as to ‘forget’ about it, then I urge you to seriously reflect on that and decide which job you would prefer to have going forward – an MSP or an MP and you should consider your position in each of these jobs and decide which is more important to you."
The Scottish Tories have been approached for comment.