The leftmost icon(rectangular) on above screen shot showed up yesterday. I don't know, where or what app it's for but I would like it gone.
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This symbol is of "Cast screen" found in Android which is used to cast your screen to a television. This symbol in the status bar means that your phone screen is being casted to a television or cast screen is active:
If you want to get rid of it, just go to settings and disable screen casting.
More info: Google Support