if the tip of the rhinophores is white (which I can't see on your picture), then I would say it is Dendrodoris nigra, which is a rather common species in the Indian Ocean.
Hard to say from a picture but if it is small (20mm span for the arms and 5mm for the body), it might be Ophiactis savignyi (Müeller -not Muelly- & Troschel, 1842)
Wahrscheinlich Culcita novaeguinae.
I agree with you. It could very well be a juvenile form of Stichopus monotuberculatus which is white and almost transparent during the first stages of its life.
Entschuldigen sie mir, ich muss mein Deutsch verbessern.
I don't want to sound pedantic about the name. It is just that I was corrected on one of my pictures by someone who really knew about the subject. In fact, the latest classification (2015) defines Cylinder as a subgenus of Conus. As a consequence, we are not supposed to use Conus.
But I am sure it will take years before this change of name is effective among aquarists. I guess lots of aquarists still use Pelmatochromis pulcher instead of Pelvicachromis. Not that it matters that much.
All the best