What is this thing?

What is this? I was cleaning my tank this morning and I shifted a rock to make it more stable. That is when I revealed what is in the pic. I think it is a majano but unsure. How did it get into my tank? I havent added anything new in the last 9 months. What is best way to get rid of it if it is harmful?

Current tank List is:
3 chromis
2 clowns
1 lawnmower blenny
1 diamond goby
1 purple orchid
1 hawk fish
1 velvet blue damsel
assorted hermits and snails
2 polyp corals (well establish)
1 glactica (well established)
1 leather tree (well established)

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