Diagnosed with ectopic hcg is going down, bleeding stopped , stable but usg shows small mass in tube can it cause infection or when will i pass tissue?

Hard to say: Infection is not a big concern. As for when you'd pass, i'm not sure if you have been treated with Methotrexate but i'm assuming that you did, but it sometimes take days/weeks for the HCG to get down to zero, that's when everything is passed.

Ectopic pregnancy: The tissue in the tube from an ectopic pregnancy is a combination of pregnancy tissue and blood and swollen tubal tissue. Over time all that tissue will be reabsorbed, so if you look again with the ultrasound in a month the mass would be gone. The HCG levels will continue to drop, and might take several weeks to go to negative. It's rare to get an infection in this situation.