Sheila T. responds:
The stringy yogurt usually indicates some contamination by bacteria and/or yeast.
contamination from the air, You can have yeast or bacteria from the air that can contaminate the milk after heating, while cooling or during fermentation.
contamination from condensation. Sometimes I have had this result when condensation created during the heating and cooling steps goes into the milk after cooling. Condensation collects on the bottom of the pot lid. Be careful to try and not let the water from the bottom of the lid get into the milk.
The fermentation temperature is too low. If too low you may have fermentation of the wrong type of bacteria which have slimy metabolites - causing the yogurt to be slimy. Bacteria are competitive.
insufficient heating and cooling of the milk. If milk isn't heated all the way then bacteria/yeast in the milk will grow while fermenting the milk. If the milk is not cooled enough before adding the starter then the hot milk may kill some of the yogurt starter,
yogurt containers are not sufficiently cleaned. They should be well cleaned and rinsed before adding milk and starter. Some people sterilize the containers with boiling water or by washing in a dishwasher.
slimy yogurt info: https://saladinajar.com/yogurt/avoid-slimy-homemade-yogurt/
I suggest throwing it out. Don't eat and don't use it as a starter.