They are known as the “bots”, short for “robots” obviously. Others more derogatively refer to them as ”pickpockets” [slow loading].
A bit more on what they do: High-Frequency Trading: Alpha Discovery and the New Arms Race.*
Some want them out of the system while others think that they are here to stay and that it’s up to the system to adapt to the new reality.
* ‘Alpha’ is a finance term referring to a stock’s performance relative to the market; it’s used more loosely by fund managers to describe beating their index – so every stock picker is essentially “seeking alpha.”