What is this guy, was on my snail

[SaracensRugby]

So got a frag and put it in my Qt tank. Then found this character on one of my snails. Any idea what it is? It’s upside down, about the size of a slightly flattened eraser tip. But he was stuck to that snail. Good bad or otherwise?

 

[EricR]

Can you take a picture from the top so we can see the shell?

 

[SaracensRugby]

So I tossed him, here is one. But looks like normal half lid snail type shell. Wasn’t sure if the leg type appendages upside down helped.

 

[PharmrJohn]

I did an image search. Based on that, it's either a volcano rock or a tumor.

 

[ISpeakForTheSeas]

So I tossed him, here is one. But looks like normal half lid snail type shell. Wasn’t sure if the leg type appendages upside down helped.

If it has legs, that would be helpful, yes - as is, it seems to be a snail; given that it was seemingly stuck to your other snail, that means it's most likely either a Slipper Limpet or a Hoof Snail.

The hoof snails I've seen in the hobby are black, so this is probably a slipper limpet - they're harmless filter feeders.