this is small sand dollar, Echinoids. Genus Loganum. But i don't know exacly the species. i found this at manado island, Indonesian too.

Vu:

Adelfia is right. Much algal growth has largely camouflaged the pentamerous symmetry.

Best

Syed

Vu Viet,

Very nice collection. 

As Adelfia and Syed said, this specimen is a small sand dollar. Echinoids.

But i not agree with Adelfia about the genus Loganum, i guess Mellita.

Best ,

Hakima

Yes I am agree with Adelfia, but the genus may be Clypeaster,  close photograph required to confirm species level identification.

Hi Vu Viet,

As I can see from the photo, considering the shape ans size, this can be a species of genus Peronella.  In Peronella, there are four gonopores in the apical disc. As Adelfia said, it may be a Laganum too. In Laganum, there are five gonopores. As well as, there is a little chance to be a Clypeaster but my gut tell that this is a Peronella. For more positive identification more close, clear picture of the specimen with scale required. 

Best regards,

Gayashan