Phyllidiella nigra fromThailand

Dear Sarawut Siriwong,
This is most probably Phyllidiella nigra. There is another species, Phyllidiopsis burni which also has scattered single tubercles on a black background, but in that species the tubercles are usually have multiple tips. In both species the tubercles are described as pink rayher than the blusih gray in your photo. I assume this is part of the normal colour variation found in quite a few species of phyllidiid, whoich often range from pink to green-gray in colour. Without more information on the external shape and colour, especially around the mantle edge, and the rhinophores, and the anatomy, I can't be totally sure of its identity. However I think Phyllidiella nigra is the most likely identity.
Best wishes,
Bill Rudman