Does a safety pilot for a PPL practicing under the hood in VFR conditions need a current medical?
As explained in AC 68-1A, a safety pilot requires a current medical:


  A pilot acting as a safety pilot per part 91, § 91.109(c), however,
  would be a required pilot flightcrew member by regulation rather than
  a passenger. Section 91.109(c) provides that no person may operate a
  civil aircraft in simulated instrument flight unless the other control
  seat is occupied by a safety pilot who possesses at least a private
  pilot certificate with category and class ratings appropriate to the
  aircraft being flown. Because a safety pilot is a required pilot
  flightcrew member, the safety pilot is required to hold a valid and
  appropriate medical certificate in accordance with § 61.3(a).


For information on a safety pilot using BasicMed see this question.

By the way, by definition a safety pilot is only required in VMC. In IMC you're in actual instrument conditions; see this question.