How secured is password protected Memo?
In general it's not a good idea to store sensitive data on your mobile phone, but back to your question.

S Memo is storing your password as a clear text in /data/data/com.sec.android.provider.smemo/databases/pen_memo.db &gt; CommonSettings &gt; PREF_APPLOCK_PIN so if your device is rooted you can use Root Explorer to read it. 
Which means that this application is not secure. You can use KeePasDroid to encrypt passwords or other sensitive data. I am using it on my laptop and on my Android device.