One of my bettas had something that looked exactly like that.
He had it in almost the same spot as well.
When he died... I was morbidly curious to see what it was... and when I pressed on it w/ the end of one of my fish nets... it crumbled in to a white powder It was definitely coming out of him b/c a little bit of blood came out after I crushed it......
No idea what it was but it was def weird.
Is the lump smooth as it looks like it in the pic.
Lumps that look like a cauliflower can be lymthoocystis. But I think the lumps can be also smooth with lymthocystis. Also it can look like a cluster of berries.