Help ID please! Good or bad!?

[mwaraxa]

I found this in my tank the other day. It's been running for 6 months now and i haven't added anything at all for almost 2, so i have no idea where this fella came from. My colt coral is pretty much dead now but it seemed like this thing was all over it. Is this what killed my colt coral? Is this thing good or bad?

 

[benter]

Looks like a sponge of some type to me... If it is maybe someone else on here can give the actual name of it..

 

[ophiura]

Compare to this:
https://forums.saltwaterfish.com/t/165371/help-photo-id
Look very similar, its a nudibranch that does feed on things like colt coral.