Wait what?
So
Your fork of cpuminer-multi with changes is OK
Fork of cpuminer-gr (which is fork of cpuminer-opt) that changes stuff and stays open source is OK - cpuminer-gr fork that I mentioned before

but
Fork of cpuminer-opt that adds another algo is not ok?

How is that logical? Your logic also makes you a plagiarist of cpuminer-multi in that case as well.
If you say that forking GPL code and adhering to GPL license (staying open source) is bad, I have a lot of questions in that case