what is this

[stillwearingaredribbon]

2 of these have turned up in my cold water tank. They sit on the glass and run up and down
Opaque looking

Mayfly nymphs (or similar- have a google of them to see if it looks familiar). They often come in on plants. If you've got big fish, they will just eat them. Some larger nymphs will eat small fish like neons or endlers, but these look like mayfly nymphs because of the three tails so they shouldn't be a problem for any fish.

I only have minnows and the intruders are as big as the fish
Will they still eat them?

They shouldn't do, I think mayfly nymphs normally eat crap detritus and whatnot. It's dragonfly nymphs you really have to worry about. If you're concerned then just scoop them out, or squish them and the fish will eat them then.