I updated to Marshmallow, and QuickMemo+ is still there (I never use it, I use the simple Colornote app). Did it get hidden? Go to your apps, select the three dots at top right and select the Hide/Show and see if it's still in the list. That is if you are stock. If you are rooted, (or where you got your phone) you may have removed the app. I guess then you could search for the apk and install it. If you are stock, Backup everything, Factory Reset and see if it comes back.
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