Kagiso Rabada was already facing censure for his behaviour after Steve Smith's dismissal in the first inning.
The masks were likely to have been worn to taunt Australian batsman David Warner, whose wife reportedly had an encounter with Williams in 2007 before meeting Warner.
Rabada shouted in celebration and approached Smith aggressively after winning a leg before wicket decision against him and the shoulders of the players made contact.
De Kock was found guilty of a level one offence under the ICC code of conduct.
Quinton de Kock allegedly made some derogatory remarks about David Warner's wife, Candice, instigating the Australian.
A footage has surfaced of an angry David Warner being restrained from a heated clash with Quinton de Kock on day four of the first Test between South Africa and Australia.
Ahead of the Test series, the Australian skipper had said that his team was looking to avenge it's 1-2 loss to South Africa in November 2016 at home.
Smith said a century in tough conditions at Centurion on Australia's previous tour in 2013/14 had given him the confidence that he could score runs against any bowling attack.
South Africa host Australia in potentially a high-power contest among two strong sides.
South Africa's batsmen will face a fast bowling attack of Mitchell Starc, Josh Hazlewood and Pat Cummins, backed by the off-spin of Nathan Lyon.
Australia captain Steve Smith will lead the Rajasthan Royals in this year's Indian Premier League.
Australian opener Cameron Bancroft suffered an embarrassing moment while fielding during South Africa A's second innings.
Steve Smith said that he has been trying to incorporate some traits from the world's best batsmen into his game.
The only likely member of Australia Test starting eleven missing from the warm-up match is opening batsman David Warner.
When Australia beat England 4-0 in their recent home Ashes series, the tourists' lower order was blown away by aggressive short-pitched bowling from pace trio Mitchell Starc, Josh Hazlewood and Pat Cummins.
Steve Smith admitted that he was absolutely ruined after the one-dayers.
David Warner is deputising for skipper Steve Smith, who was rested after the One-day International series against England.
Australia cricket captain Steve Smith capped off a golden 12 months by winning his second Allan Border Medal in Melbourne on Monday.