I’ve found a little lump on my dog and when I squeeze it gray-like goo comes out of it. What is it?

Lumps of this sort are usually a type of overgrown sebaceous gland. These lumps are often, but not always, benign. Clean the lump with water daily until it has healed and make sure the dog cannot lick it; it may be wise to put a buster collar on your dog to prevent it from doing so. Consult a veterinarian if the lump returns or if your dog is agitated by it.