Yeah looks like some pest nem to me like some kind off aiptasia but lots and lots off kinds that often get bunched together.
Could remove snail from tank and put In Container off tankwater and carefully remove it,making sure to get all pieces off it and gently swoosh snail in another container off tank water ( gently as dont want give brain damage ha ha)
Lots removal methods from injecting boiling water,vinegar,kalk paste into mouth plus others,but most importantly remove all off it.
Seen
@Eagle_Steve give some good advice on pest news and names of species,maybe he can enlighten you little more .
If on one snail shell depending how long it been in tank,maybe others on your rocks elsewhere who knows so maybe have good luck around with zoom on your camera as some so small
It does look like a little baby aip or exaip (we will not get into the differences lol)
On a snail, this is super easy. I will say, with it being a baby one and if the snail is not new to the tank, you more than likely have a few more hiding somewhere. No need to flip out though. You can just remove them as you see them or if they do start to get bad, there are some other options to eradicate them.
OK, back to removing from the snail.
Take 2 glass bowls or 2 larger cups (either is fine) and scoop some water out of the tank.
get an old tooth brush and some 3% hydrogen peroxide (don't need all of the other stuff for this in this situation)
Replace water you just took out with new water.
Take snail out of tank, but be sure not grab snail where the nem is.
You will then apply a little hydrogen peroxide to the area where the nem is. This can be done with a q-tip or small makeup brush.
Once that is one there, you will scrub that area with the tooth brush to remove the nem. After scrubbing set the toothbrush in a small bit of h2o2 while you do the rest of the below.
From there, rinse off the snail by dumping water from the first cup over the snail while holding it over the sink or a bucket or larger bowl. Whatever works, as all of the water you took out will be disposed of. Be sure that you are rinsing the snail off where the snail is "upside down". We do not want any water or left over nem to wash into where the snails resides lol.
After that rinse, apply some more hydrogen peroxide to the entire snail shell with a qtip or small makeup brush and be sure you are only doing this to the shell.
Let sit about 2 minutes, as we want any gametes, sperm or bits of left over nem to bust/degrade from the h2o2.
Then scrub the shell with the toothbrush again.
Then rinse again the same way you did before with the snail "upside down".
Place in tank and all should be well with the snail.
Also, as mentioned before, do not fret if see more of these guys. If you do, make a post with pics of them, so we can see where they are located. From those we can help you come up with a solution to remove them.