my silver tip tetra has a lump and redish marks on the lump, what could it be?

You don't have any other fish that could interbreed with it. It may be Neon Tetra Disease, or False Neon Tetra Disease. Other Tetras other than Neons can catch it. It is characterized by lumps, red sores, and finally loss of appetite and inability to swim and if it s Neon Disease it is fatal, If its false Neon disease, it can be treated with tetracycline 250 mg/10 gallons of water. It may just have a tumor, either cancerous or benign as well. Since either of the Neon diseases are contagious if the fish is pecked at or eaten, you should isolate it. time wil tell, if its Fase Neon you can cure it, if it's true Neon Tetra Disease it will die.

You should probably treat with tetracycline anyhow to prevent infection even its is caused by something else. Lots of lumps hold parasites or bacteria. The parasite could range from Pleistophora protozoans which cause Neon disease, to Flukes which can be killed with Prazipro. It's still fairly early, so if the fish is valuable to you I'd try tetracycline and coppersafe both. thus eliminating most things except viruses.