Unknow mob

I was on a server in a Vanilla Minecraft Java There was Only Another guy on the server, but...

I found some particles In my Underground base It's near an AncientCity The particles were

"Invinsibility&Strength" I got scared... But when I told the Other guy about He was In his base

There were also footsteps and I wasn't moving The other guy said "I don't know" Also When I Started talking about It the Mob disappeared.. I couldn't screenshot it..