If the aptaisa just stayed as one, then yes, they are kinda cool. But no... that innocent looking thing in your sand will dominate your tank if left unattended. They are the weed of saltwater.
The added nasty part is that they are one of the stronger stinging organisms you'll have in your tank, so they make some heavy grief for any corals.
And.. they are not easy to remove once established. Hence why Laura D mentioned about scooping it out of the sand. If they root into rock, they suck themselves into crevices when they sense trouble.
Also... any form of trauma to its body and it expells spores into your water stream which makes it multiply.
Fortunately, there is quite a bit of info on the website about these critters and how to get rid of them. You definately want to remove it.
You can use Kalkwasser paste injected into it base, or Joe's Juice (a marketed product found in many LFS) squirted into its mouth. Some have claimed success using vinegar and boiling water injected into them. Also, Peppermint shrimp have been known to munch on them, but some say it is a hit and miss. If you use injection methods, be sure to turn off your water flow.. as mentioned they reproduce exponentially when you traumatize them, so keep your current from spreading the spores.
For you however, if it is still in the sand simply scoop it out... before he wanders over to a rock and becomes tougher to nab.