Crab on my Acropora Tenuis

[Billyreef-ita]

Ooops…talked too early…
In the sump I have a refugium with chaeto, sand and also much life (pods and nails). I hope he finds his way there

 

[ISpeakForTheSeas]

Looks like a Cymo sp., possibly C. andreossyi or C. melanodactylus (this one if it has black claws):

The crab may be a Cymo sp. such as Cymo melanodactylus - they're obligate commensals with Acropora corals, and their relationship with them seems a bit mixed; they reportedly do very, very little damage to the coral, and they seem to help keep the corals healthy if the coral is diseased.

That said, there are also reports like the one linked below where the crabs may be responsible for killing coral colonies.

Are infestations of Cymo melanodactylus killing Acropora cytherea in the Chagos archipelago? - Coral Reefs

link.springer.com

However, the corals dying may have been caused by a few separate things:
-Overpopulation of crabs
-Disease
-Bleaching
-Etc.

If the corals were dying for reasons unrelated to the crabs, however, then the crabs may have been trying to protect the healthy colonies by consuming the sick/dying ones.

So, the crab is essentially typically a beneficial parasite; if the coral is small or struggling in your tank, then the crab could very well kill it, but if it's large and doing well, it should be fine to keep.

 

[Billyreef-ita]

it’s definitively him!
The coral is small, it seems to risky, now the crab is in the sump.
Thanks