What is this?

[Mech Eng]

I found this in my tank this morning. It's about 1" long (max).

What is it? It looks like some kind of slug. Looks kinda cool with the colours.
Is it reef safe? I don't have any corals yet, but would like to in the next month. Tank has been running for about 2 months and has 3 fish in it.

Minatus Grouper
Magnificent Foxface Rabbit Fish
Maroon Clown

110g with 120 lbs live sand, 70lbs live rock (more coming) as well as some other "dead corals" that are starting to grow algae on them.

Mg is a little low as is Calcium (Mg 1230ppm Ca 400ppm dosing to raise)
Kh 8
Phospates 0
Nitrates 0
Ph 8.25



Thanks