Just noticed this on a snail making a trip across the front glass... Had a quick check around the forum + Google. Something common, good/bad? Photo color is an accurate representation.
Vermited snail. They are not the worst thing to have in your tank and everyone gets them at some point. They reproduce prolifically in a well fed system. During feeding they send out a slime web to catch food and retract it back to their tube. This web can irritate some corals especially if there are a lot of them. I would pull the snail out and with a razor or blunt object pop it off the snails shell. Be sure to get all of it off. They commonly are attached to frag plugs as well so it's wise to remove plugs if you can or rasp the plugs before putting into you tank. Dips won't kill them.