It looks just like ordinary milk from the real world, except that it is solid by the time Patrick has it, due to it spoiling.
Role in episode
After Patrick wails at the mention of the Dirty Bubble challenge, SpongeBob invites him to some kelp cookies and a glass of seahorse milk.
The milk is solid and SpongeBob then trips over Gary's food bowl realizing that he needs to feed Gary. He then gets the bag of food from outside and calls him for dinner but when he does not come, SpongeBob asks Patrick how long he was taking the Dirty Bubble Challenge. Patrick, judging by the condition of the milk, which has spoiled to the point of being completely solid, tells SpongeBob that he was gone for about seven to ten days.