Little white worm

Germinated a seed the way Robert recommends , in a glass of water. Tail came out as it always does. Then I put the germinated seed in a starter plug, from root riot. Its like a small sponge. The plant has broke through and is growing. Took about 3 days to sprout. Now I’m seeing a little White worm that pops his ugly ass out of the sponge and then disappears. Now ,I been growing with these plugs for a long time now ,with 100% success rate and I’ve never seen this type of worm before or any worm for that matter. The plugs are kept in the refrigerator in the crisper. Where did this worm come from and more important how do I Kill It. Any suggestions would be appreciated.