He probably was hiding in the plants.
Thats how I got my snails in there too (who just laid eggs btw

Snails are very easy to keep happy; they nibble a bit on the plants and sometimes I throw in some lettuce or cucumber.
However with your fry that might not be wise so maybe you should move him out into a little container?
Once your fry are less sensitive to mucky water you could move him back.
They dont hurt each other, though the fry tend to try the snail out to see if its food