Occasionally on Twitter, I see tweets from young native Japanese-speakers like:
with the obvious meaning of おはようございます。
Is this change of ます to まつる just a form of slang, and is it commonly used and understood?
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As mentioned in the comments above, this seems to be just a small Twitter following. The main user, YANAKIKU, is a musician, and changing the ending of verbs to 〜ま鶴 seems to be their usage of artistic license, most likely to be cute.
I haven't seen it used anywhere other than this, so I assume the meme (if it can be called this) is really just for fans of the group.
Other examples from their twitter page include:
マシュー今日はありがとうございま鶴でした！マシューのMCだとなんともいえない安心感がありま鶴！ めっちゃかっこよかった♡ 最後ご挨拶できなくてごめんね！これからもよろしくお願いしま鶴！
Not really something I'd be able use anytime or anywhere without feeling self conscious ; )
Commonly understood, not commonly used. It sounds cute or annoying, depending on your personal biases. It has no special meaning.