it was in about 15mtrs in water off lombok indonesia
sorry the photo isn't better , it was strange that it kept rolling itself up length wise then
rolling sideways a couple of times then going forward a bit and then doing the same again
it was about 2-3 inches in length
thanks in advance
vinny
Looks like a Marionina distincta, but is a bit darker than the photos in my references. There are two patterns found; the other form has horizontal lines.
Here is a link to the darker one which yours resembles: