Hi Dear, Welcome to HCM. Understanding your concern. Thanks for your query .The pimple or lump above your decayed tooth is clear indication of severe infection in tooth .When infection travel from crown of tooth to the root tip it form abscess there and pus come out from that abscess through lump or pimple on gum by making sinus . You should not ignore it as infection can spread further to the jaw bone and in surroundings . I would suggest you to visit dentist as soon as possible .Dentist may order X-RAY for proper evaluation to decide the line of treatment.If the condition of tooth and bone under it is sound then he/she can drain pus from it and perform root canal otherwise extract the tooth ,also prescribe antibiotic course. For now what you can do is take over the counter painkillers like ibuprofen or paracetamol and do warm saline rinses which will help in drainage of pus. Hope your concern has been resolved. Get Well Soon. Best Wishes, Dr. Harry Maheshwari