What are these things growing in my toilet and what to do?

Kind of a long story, but would appreciate any help! This was the only toilet we didn't change during a home remodel 5 years ago. Toilet wasn't used for about 9 months of remodel. Afterwards, toilet had a tendency to develop a sewage smell and grow black mold on the seat and inside of lid if unused (it's in guest bathroom). Cleaned and disinfected with Lysol and bleach a few times, but the sewage smell and mold still comes back sometimes. I made a point to flush the toilet a few times month... But forgot in last month. Then saw what I posted in the picture. Are these tiny worms?

And any ideas on the underlying issue that's causing these problems? And what the remedy would be.

Thanks for any help!!