Don’t have invasive grasses but do have lots of algae. Have introduced trapdoor snails and crayfish. Have read where in a small pond that GC will hinder not help. Thoughts?

What type of algae? Planktonic, filamentous, chara? GC will not manage any algae with exception of chara which is a macro algae resembling coontail.

Honestly can’t tell you. It’s green.

We all start more or less in the same place - 15 years ago I wouldn't have known the difference either. I suggest performing some research on algae types and match to your conditions and report back. I suspect you have filamentous algae mats - GC will not manage. Dense populations of crayfish can make a difference, but those are hard to achieve in presence of LMB. I personally rake mine and remove from the watershed and create potting medium and mulch for my garden. Treating with algaecide merely continues the cycle. Manual removal isn't fun or easy, but it's one of the best long term solutions. Establishing beneficial macrophytes [submerged and emergent vegetation] will utilize nutrients otherwise headed towards FA growth. Combine manual removal and getting beneficial macrophytes established will result in the best long term management of FA.

If your algae looks like cotton candy or stringy hair in water then you have filamentous algae FA. For no or minimal chemical control use tilapia with around 30-40 lbs per acre are often suggested. In light infestations of FA you can use fewer tilapia of around 20lb/ac or for heavy amounts of FA it may take 50lb/ac. It is usually best to try and reduce the amount of FA before adding the tilapia so they are not overwhelmed by too much FA to eat when you stock them. Too much and too few tilapia and lack of FA control is often the result.

For other methods of dealing with it read through and study the info in this thread from our Common Pond Q&A Archives.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92633#Post92633

A pond with lots of FA basically means the nutrient concentrations are out of balance and too few natural plants are present to compete with FA. Higher plants and FA compete for the same nutrients. The most abundant one thrives abundantly often to the point of nuisance.

Dad and I put grass carp in his 2 acre pond about four years ago and have not had any type of algae at all in the pond.

GC do not utilize FA as forage.