Not sure about the lump but if they will continue not getting water changes they might get sick later or sooner. Even for 10 gall tank you need to do 50% water changes a week with the gravel vacuuming , and the filter media swish/rinse weekly in the tank water.
Lamp is it looks like white like a cauliflower then its might be a lymphocytes, which is virus and usually not a fatal unless its get infected. And it can get infected if water is not super clean.
Or its might be a tumor. I think if you will post a picture it might help
Betta fish pictures with lymphocystis