For a garden plant I would just wash them off with a sponge, water, and maybe a little dish soap. The bigger problem for citrus is if you don't control the ants you WILL get aphids. It is almost always true that when you see aphids on citrus you have an ant problem. No ants, aphids are rarely a problem.
I agree with rhizo, they look like wooly aphids.

Thank you,
JOhnmerr and Rhizo 1 for your replies.
I have never had this problem before. The grapefruit tree is 8 years old and the lemon tree is 2 years old.
Can these "wooly aphids" kill our trees?
I will look up how to treat unless you have suggestions. The grapefruit tree is way too big for me to sponge off with water and soap. It's about 15 ft. tall
Ants have been very bad herein Texas this year due to the drought last year and the past warm winter then lots of rain in the early part of the year.
Geeez...ants have been everywhere. I have been treating them with orange oil and molasses.
Should I prune back some of the branch's to allow more airflow or is that not an issue?

Thank you again ;)

I have the exact same problem and have not yet discovered anyone who is sure of what this problem is. My tree is still small so i shot with a power garden hose the underside of the infested leaves. Will update in a few days.

After research the first guy is correct wooley aphids. Use dish soap 1 tbl spoon to a quart of water. Or you can buy a insecticidal soap.

this is not wooly amphids, if it were you'd see living creatures at some point, i've seen none i've had this problem for years, treated whit almost everything, going to try a systemic fungicide now.

This is a whitefly infestation - probably Wooly Whitefly, Aleurothrixus floccosus, although some other species look similar. The white deposits eventually become black with mould but it's no use trying to control with fungicides - you have to get rid of the whiteflies. Not easy without using nasty chemicals.

Try neem oil. I have the same issue and I'm going to try this natural, safe solution. Good luck!