SMBH formation is a mystery because quasars suggest energy from supermassive black holes (SMBHs) at very high redshifts when the universe was quite young, which the Eddington limit would seem to prevent. Surpassing that limit during such a black hole's growth would likely involve detectable phenomena and so far, no observations have produced likely candidates for such detections. It is an area of active research, with a number of theories of what might contribute to their presence when the universe was so young: how such a black hole might start with a very high mass and/or how it might grow quickly in some manner that we fail to observe. SMBHs today have had sufficient growth time and their growth is not such a mystery.
The term black hole seed refers the black hole's progenitor (or, for an SMBH, "seed" perhaps may refer to a smaller black hole from which it grew) and some theories involve proposed black hole seeds with masses much higher than stellar-mass black holes:
Subsequent rapid growth might include mergers (though the final parsec problem suggests that might require some explanation) and accretion if it tended to be hidden for some reason. One theorized factor is radiative inefficiency, e.g., assuming the accretion disk to be an ADAF in which substantial energy falls into the event horizon before it is radiated away. Another is obscured luminosity, i.e., electromagnetic radiation from the accretion process is blocked from our view by gas and/or dust.