Jelly does not form in pure water.
Water that is not pure may well contain some bacteria and some amoebas that may reproduce.
As part of a the industrial scale purification of water, chemicals are added that encourage impurities to form a jelly-like sediment that can be filtered out before the water is provided to users. Other chemicals (usually chlorine) are added in minute quantities to control the growth of bacteria and amoebas.
Can you give more information about the water you are asking about?
Are you thinking of frog spawn, maybe?
Jelly forms in water
