Phone has a green alien and says

My Samsung Galaxy S2 Skyrocket , indicated that the battery was low. So, I powered it off, plugged it in to charge it... and a message popped up witha picture of a little green alien,
it says:
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factory Mode
Product Name: SGH-1727A
Custom Binary Download: YES (1 Counts)
Current Binary: Custom

pic of an alien green

downloading......
Do Not Turn Off Target!!
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Important to know. I was just given the phone by my son a couple weeks ago. It does not have a sim card in it yet. He kept his for his S4. I've just been playing the games and emailing over the internet thus far. Hs worked well up until this morning. Its been like this for around 8 hours now, so I doubt it's downloading anything. I'm afraid it will kill my battery eventually. Has anyone had this happen? Is t a virus? Is there a fix you could post for a newbie, (by that, I mean explain it step by step. Its my first ever cell, and I have little to no clue.) Thhanks in advance for any knowledge you'd be willing to share.
Sue