A red hard-shelled organism

[Yaozhen]

Hello everyone, I found a creature on live rock that I’ve never seen before. Could you help me identify it?
Its characteristics are: a red shell with spiky edges (similar to a crab shell), it extends tentacle-like appendages like a tube worm to catch food, and it also seems to have mollusk-like tentacles.
（My English is not very good. I used a translator for the above. Please correct me if there are any mistakes.）

 

[Yaozhen]

Also, this is not a new rock; it has been in the aquarium for about five years.It grows in the shadows, so it’s hard to take a clear picture.
This is my aquarium.

 

[logibear]

Bump @ISpeakForTheSeas

 

[Crabs+Shrimps-69]

Could it be an acro eating flatworm or red bug?

 

[Yaozhen]

Could it be an acro eating flatworm or red bug?

It doesn’t move and currently seems more like a tube worm, feeding on the plankton in the tank.

 

[Yaozhen]

我尝试拍摄一张更清晰的照片。

 

[Yaozhen]

I think I may have found which species it belongs to，“Spondylus”
But it’s still a little baby, so it’s difficult to determine its exact name.
Anyway, it’s still an answer, so that’s progress!