The white thing is some sort of a mushroom, named onifusube(オニフスベ). Reimu asks it to take it easy but whether or not it's really sentient, it can't reply so Reimu is confused. The mushroom is known to spread its spores that grow pretty fast without sign, so apparently a spore stuck on Reimu's top and got the guy wondering whether Reimu got hit somewhere and grew a bulge. Well I wouldn't say it's not family friendly, given the light-hearted atmosphere here.