I was recently informed that there is a minhag (custom) to not visit graves on chol hamoed.
Does this apply to visiting the graves of tzadikim?
There are a whole bunch of Poskim that answer this question online.
Rav Eliezer Altshuler says that people go to Kivrei Tzadikim to pray, which is always allowed, and that there are different customs about visiting them on Chol Hamoed and during the month of Nissan in general.
Rav Uziel Eliyahu says that adding a visit/prayer at Kivrei Tzadikim during a trip/Tiyul with no other Jewish content is a good thing to do, and notes that there are those who have a minhag to go to graves of Tzadikim
Rav Dov Lior says that it is accepted that we do not go to visit any Kevarim on Chol Hamoed, even for a Yahrtzeit, and that therefore one should not visit even Kivrei Tzadikim.
The Rabbanim at "Din" say that it is permitted to go to Kivrei Tzadikim, and that it was not included in the Minhag to avoid graves on Chol Hamoed.
Rav Binyamim Shemueli says that one should not visit Kivrei Tzadikim on Chol Hamoed.
Rav Baruch Blizhinski (quoting Nitei Gavriel Aveilus 83:4) says that visiting Kivrei Tzadikim should be avoided unless there is a specific need to go Daven for someone ill or something like that.
I'm sure that there are many others who address this, and a personal Pesak should definitely be asked in each case.