Equihash 125_4 (or ZelHash) is a mining algorithm. It is a modified version of the standard Equihash algo, and it is used mostly by ZelCash cryptocurrency. The developers of ZelCash switched to the so-called ZelHash in the summer of 2019. The goal was to protect the network from ASIC and FPGA once and for all. It is important to note that cloud mining is not supported by Equihash 125_4, so you can’t rent power and mine ZEL. In theory, the network seems truly protected from external influence.
Please note that you can not use ASIC-miners for Equihash on Equihash 125_4. It is a different algorithm.