Mystery deaths!!!

[Mjb51]

Hi having a problem with my tank, have lost my flame angel one day ago fed him on the night he was fine eating and swimming fine got up in the morning and he was dead on the sand bed half eaten! Which I presume may not have killed him as hermits in there may have started eating once he died now have found my sand sifting starfish with its tips missing (see photo), he is still alive but will he recover?
In my tank that I have purchased is yellow tang
Clownfish
Regal damsels
Six line wrasse
Bristletail filefish
Hermit crabs
Collector urchin
Various snails
2 peppermint and 1 cleaner shrimp
My water parameters are
Ammonia and nitrite 0
Ph 8.1
Nitrate 0.1
Salinity 1.024
Calcium 490
Magnesium 1470

As for hitchikers several bristle worms I believe it to be an acro crab? (Best pic i could get)
Anyone any suggestions what may be causing these losses? any suggestions will be appreciated

 

[Dav3]

What size tank? How old?

Starfish probably doesn't have enough food so it is slowly decomposing.

 

[Mjb51]

It's a 260 ltr tank, the tank has been running for 15 mths after cycling, the starfish had been growing well almost doubled in size since we put him in about a year ago,

 

[Dav3]

Any changes to the tank on the past 2 months?

 

[Mjb51]

I added two black footed turbo snails about three weeks ago but other than that nothing has changed, the only other thing it might be is when it was cycling I found a couple of eunice? Worms they look like a cross between a worm and a leech with five small tentacles on the front but I haven't seen one for over a year, might have to get up in the middle of the night and see if Anything is out.

 

[Mjb51]

Would this be the culprit?