Unknown snail spawning?

[Mr.nintendo]

I woke up this morning and sat down to look at my tank and noticed three snails of an unknown type spawing in the tank, on one side were 2 males and the other a female. The female was spitting mass amounts of eggs into the tank well the two males produced clouds of sperm.

I have never seen these snails before, they were all grey brownish ( couldnt get a picture my phone is to low quality to differentiate the snails from the rock )

The snails are more slug like with small shell sections that barely stick up from their body, only covering just behind their head and midsection.

Any idea what these guys are?