Salmon does contain mercury but the mercury in Salmon is lower than other fish.
And you would have to eat a huge amount of Salmon to get mercury poisoning from the Salmon so it's really not a problem.
As long as you don't eat too much Salmon or eat Salmon everyday then you should not get mercury poisoning from it.
Salmon is very good for you in moderation but just don't eat too much of the Salmon too often as in everyday and you'll be just fine.
Some people do eat Salmon everyday or nearly everyday and they are fine as well but if you're worried about mercury poisoning then just eat the Salmon in moderation and there will be no risk of mercury poisoning.