After comparing it to the pictures in the link below it sure does look like a Rissoid snail. The fact that it's on your clam would make me a little nervous though.
Thanks Mike. I've just been reading up on pyramid snails and it does look dodgy. The snail is same place on one of the scutes as it was on Thursday night so seems suspicious. I found this link.
Thanks @mikejrice and @Oxylebius. I picked my clam up today and held her close to the water surface and had a good look at her (she was underwater the whole time). On the side I can see easily in the tank there was one big snail (in the photograph) on the other side 2 more big (5-6mm) snails plus lots of small ones. So out came the forceps and I picked off 24 snails. Will check again every few days. I could see no eggs but I can't believe they were not breeding.
On the plus side it looks like she has grown new scutes since I have had her. Hopefully this will perk her up a bit more.
Thanks Sue. You always find the interesting reads! I'll give the clam a good checking over every couple of evenings now for a few weeks. After not seeing any Aeolids for a few weeks, I've not had my hands in the tank so much so I'm missing my cleaner shrimp manicure.....
On the subject of the cleaner shrimp I think mine is ready to drop its eggs again. They are going browner and I was watching the shrimp grooming the eggs whilst I filled the ATO just now. Feeding frenzy for the fish anytime soon!
I removed 6 snails from the clam in the middle of the week but there must have been an egg mass I missed as today I removed 43 tiny snails. I'm not seeing any big ones just really, really small ones. The clam is looking better though with more mantle extension than before.