Are decentralised services powered by network hash power
A decentralized service is a service which is being provided in a decentralized network (e.g. peer-to-peer network).
This has nothing to do with "hash power" or Proof of Work in general.
For example btc is decentralised however if a miner or a single group of miners have over 51% of the hashing power of the btc network then that means they can launch a 51% attack on the btc network right?
Theoretically, yes.
Can same be said for decentralised services? Which decentralised services are immune to 51% network attacks?
All of those services which do not rely on mining etc.
A 51% is an attack against cryptocurrency.