Is this a good or bad hitchhiker?

[Starganderfish]

Hi all,
Found this thing clinging to the underside of my mesh lid. It was in a fairly new setup 20 gal tank, used dry rock and dry sand and cycled with Ammonia and some water from my 4 gal Pico. I've added two clowns, two coral banded shrimp a couple of Trochus and a heap of Nassarius and Turbos. Corals are a Goniopora, a Plate coral, a Hammer coral and an Elegance coral, all were dipped in Seachem Reefdip.
I expected there'd be a few small hitchhikers but this thing is about an inch long?
It's a dark brown mottled colour and a hard, rough leathery texture. Lozenge shaped, about an inch long. The underside is flat like a snail. It has this row of hard ridges or large scales running down the centre of it's back. I popped it off and it immediately curled up on itself into a little ball. I left it in the dark for half an hour and it's since uncurled.
Currently have it in a small covered cup of tank water.

I'm not at all expert ID'ing creatures so not really sure what it is?

Easy enough to chuck it away if it's bad but is it likely to have laid eggs or infested the tank?
Appreciate any help ID'ing this

 

[Tonycass12]

chiton snail. Good guy and nice addition to your cuc.

 

[Starganderfish]

I thought it might be a Chiton but a lot of the pics online of those looked a lot more obviously segmented, like trilobytes or something, so I worried it was a Polyclad worm or something. But it has the eight scaled ridges on his back and is pretty hard when its tensed up so looks like your right.
Glad to hear he's a good dude. Doesn't want to leave the cup though, stuck there like he's superglued on. Just put the whole cup in the tank and hopefully he crawls out on his own
Thanks for the help