Video circulating on social media appears to show two people in balaclavas approaching an entry before a large blaze spreads.
The footage has not been verified by the BBC.
There is renewed focus around safety at places of worship following the attack outside a synagogue in Manchester on Thursday, in which two Jewish people were killed.
The suspect, who was shot dead at the scene, has been named as 35-year-old Jihad Al-Shamie, a British citizen of Syrian descent.
Tariq Jung, chair of Brighton and Hove Muslim Forum, said “everybody is deeply saddened and appalled” by the events in Manchester and Peacehaven.
He said that, irrespective of whether worshippers were Jewish, Christian, Hindu or Muslim, the “last thing” they should expect is that someone would try to harm them.
“I hope faith leaders and others within the community will come together to try and bring peace,” he told BBC Radio Sussex.
“We’ve got to… support each other at this time of great need.”