what is on my Euphyllia

[davejnz]

I have a
Euphyllia divisia a.k.a frogspawn that has little flat circular shaped organisms living on the polyps.They resemble the tips of the polyps
therefore they went unnoticed for months until i seen them moving.They
are cream colored and have little lines in them.Are these harmful or
do they have a symbiotic relationship with Euphyllia corals?

 

[Emerald]

They certainly sound like flatworms. If they dont appear to be chowing on the coral directly, they may be just thriving on the mucus it produces. The biggest worry may be that they starve the coral of light. In many cases they will eventually disappear on their own. If they have been there for quite some time you may decide to do something about it. You could try stronger water flow, not enough to damage the coral though.
There are few species of fish that may eat them but not guaranteed.