Noncitizen immigrants who reside in states that have decriminalized marijuana must be wary of using and possessing marijuana even when it is according to state law. It's still a federal offense to possess marijuana -- federal law governs our immigration system. For example, a noncitizen could be found to be inadmissible or deportable just by admitting to the possession of marijuana. Moreover, it could affect whether a noncitizen can successfully become a citizen.
The best strategy is to not use or possess marijuana until you are a citizen, and if you need medical marijuana, get legal advice, do not carry or show any marijuana related items, remove all information or photos from your social media relating to marijuana, and never discuss marijuana with immigration, agents at the border, consular officers, or law enforcement. If you are a noncitizen and have ever used marijuana, consult with an immigration lawyer before leaving the U.S. or applying for any immigration benefit.
But what about noncitizens who already have a misdemeanor marijuana conviction in a state where it is now legal to possess marijuana? Luckily, if you have a conviction from the City of Seattle, your conviction will soon be vacated. That means it will be like the conviction never happened. The city of Seattle will move to vacate misdemeanor marijuana-possession convictions prosecuted by the city before pot was legalized in Washington -- https://www.seattletimes.com/seattle-news/seattle-to-vacate-misdemeanor-marijuana-convictions-dismiss-charges/
Washington legalized marijuana in 2012, but there are many noncitizens that may have misdemeanor convictions from before that time. If you are a noncitizen and have a misdemeanor marijuana conviction in from the City of Seattle, it is important that you make sure your conviction will be vacated. The first step would be to get copies of your criminal record regarding the marijuana conviction. Then contact the prosecutor that was assigned to your case. The prosecutor should be able to give you information about the process and if you need to do anything to assist in the process. Even though the City of Seattle is taking the lead on finding and vacating these convictions, a noncitizen immigrant with a misdemeanor marijuana conviction should be proactive about making sure it happens.